I know in a scenario like a commercial restroom with occupancy sensors where a light may be turned on a hundred times a day you're not supposed to use instant start ballasts because starting is hard on the lamps but should instead use programmed start or rapid start. What about a situation where a lights may be turned on a dozen times a day. What about a half dozen?

I'm looking at buying some more T8 fixtures for my basement that come with instant start ballasts so I'm wondering if I should change them out.

The best scenario for instant start ballasts are to be used on fixtures that are turned on once a day and left on or just very infrequent use.  I've run some test fixtures myself at my church buildings.  Coming up on 3 years of daily use and the tubes on instant start frequent use are going EoL right now where as the ones on the programmed start ballasts are still going strong.  One of my buildings built in 98/99 had the heavy brick type rapid start T8 ballasts, and I've yet to have any ballast failures on those and the tubes do last a bit longer vs instant start. 

Any type of lamp is ideally best suited to infrequent switching. All starts and restarts will eventually reduce lamp life, so it's a trade-off in most cases.

Certainly pretty much any discharge lamp (including fluorescents and CFL's) will last longer if switched on and left on for a few hours before being turned off. Even incandescents will thank you for not constantly switching them as well. Don't really know all that much about LED's as I try and keep them at arm's length..!

Any type of lamp is ideally best suited to infrequent switching. All starts and restarts will eventually reduce lamp life, so it's a trade-off in most cases.

Certainly pretty much any discharge lamp (including fluorescents and CFL's) will last longer if switched on and left on for a few hours before being turned off. Even incandescents will thank you for not constantly switching them as well. Don't really know all that much about LED's as I try and keep them at arm's length..!

Fluorescent:
If the preheating is done correctly, the starting wear becomes comparable to less than a minute of continuous bern time. If you add the electricity cost for that time, the break even point becomes few 10s of seconds. So if such setup is switched off for longer than a minute, it will actually last longer. Of course, there are huge differences in ballasts performance, generally those using longer preheat time with not that boosted heating power tend to wear the lamps the least.

LEDs:
These age mainly by temperature (generally each 10degC hotter means rwice as fast wear), then the current (higher current means faster wear, but compare to the thermal effect that is very minor). But because some wear mechanisms lead internal leakages (a kind of parallel resistor), at lower DC current the wear leads to no light or color shift faster (that is, why dimming is virtually exclusively implemented by PWM and not DC current reduction - as the constant peak current ensures even the worn out LEDs are able to still operate)

Ballasts:
But with both there is certain ballast wear, mainly the power on surge (regardless if it is the charging current spike with a capacitor filtered rectifier in electronic, or a magnetic "thud" causing wires to rub each other in the winding dur to electromagnetic forces).
For frequent switching is then better to have the light sensor switching done as part of the ballast functionality and not switching the power to a ballast by the sensor (then there is no inrush, as the power input is always on).
With LEDs the thing is a bit simpler: Because they dont need any high voltage, nor have any problems restriking, for frequent switching with separate sensor is better to look for designs without the input capacitors. The good signature for these is their 120/100 Hz flicker (it is the in the end the smoothing filter capacitor, what makes the reliability problems).

But generally a good quality ballast should outlast the life of the relay in the sensor...

Incandescent lamps can be switched frequently and be used in flashing lights and motion sensors. Their life aren't shortened because of frequent switching like discharge lamps.

Incandescent lamps can be switched frequently and be used in flashing lights and motion sensors. Their life aren't shortened because of frequent switching like discharge lamps.

Not they wont wear on power cycling at all (they do wear out faster at power ON, because of the local filament overheating, mainly the mains halogen are sensitive on this), but the cost related to keeping them ON instead (normal wear, electricity) is so high, the break-even point of off time duration is shorter than it takes the filament to cool down (so when shorter, the warm filament increased resistance suppress the overheating stress).

Connecting an Incandescent lamp in series with an 1N4007 diode prolongs its life by a factor of ~3x..5x - this hack had been used a lot in the past in staircase lighting etc. Yet there is very visible 50Hz flicker, and i think with low power lamps (25..40W) the minimum temperatue for the flicker cycle is quite low. The lamps didn't seem unhappy about the constant flicker at all

Ash - I reckon maybe a diode and filter capictor.

Connecting an Incandescent lamp in series with an 1N4007 diode prolongs its life by a factor of ~3x..5x - this hack had been used a lot in the past in staircase lighting etc. Yet there is very visible 50Hz flicker, and i think with low power lamps (25..40W) the minimum temperatue for the flicker cycle is quite low. The lamps didn't seem unhappy about the constant flicker at all

Well, at the same time it reduces the efficacy to less than 50%. So you need more than double power consumption for the same output.
With the cost of electricity (both financial, as well as environmental - including release of mercury from the coal to the atmosphere means more than if you break one CFL for each incandescent) it is one of the dumbest ideas.
The thing is, unlike the dedicated long life or high reliability lamps, the diode still makes the filament to suffer from cold power on stress, so the the cycling life is not that much better than without the diode (the filament overheating during warmup is not much different, I mean wattages in the 40..100W range). So if you need long life or high reliability, way better idea is to specify really a high-rel or long life lamp. Those have just 15..30% efficacy penalty compare to "standard" spec lamps, yet they really do reach the extra reliability or longevity needed in such application.

Such diode is useful only to prevent complete cooldowns (so repeating turn ON stresses) during "off" phase when the lamp is used in a flasher or similar application, where with just 30% extra power consumption it prolongs the lamp life many times (and the efficacy could be offset back by the use of more efficient halogen for such application, the diode prevents the stress what uses to be killing for the halogens).