Yep...is this an actual failure mode?

Have a circa '93 Fisher-Pierce photocell that is doing this; it turns the light off after the 2 minutes (light was on a switch), even in the dark!  It used to be a very insensitive PC so I highly, highly, highly doubt it was "seeing its own light", plus it didn't seem to cycle on and off. 

I tried a new bulb, so I know it wasn't a cycling bulb either...

Thoughts? 

If it is supposed to be just a basic photocell with a bit of extra filtering time as the only advanced feature, it really would be some fault inside. If it is a voltage compensated thermoelectric relay type, the auxiliary heater (the one doing the voltage compensation) may be broken.
Or mainly if it is someelectronic one, there may be some leakage across the sensor (humidity damage,...), so the electronis "thinks" it is a day.

If it is some advanced one like anti cycling or part night only, it may became erratic after power application and may need to be left alone with power ON for at least one day cycle before it starts to function normally.