Hello to everyone.I need your advice.My friend wants to make an artifical sun lightbox which will be used to see how resistive are plastic products against UV.What kind of lamps should be used and how many to replicate sitting at the sunny day for a longe period of time,like one month?

Does he need the test to be accelerated (ie a month in the box is supposed to simulate year of real life or longer, so the intensity to be significantly greater than the natural sunlight)?
If not, just put the thing outside on the sun...

Yes.He wants acelerated test.

I would have thought a simple and easy way would be to use a mercury lamp with the outer envelope broken.
This will kick out a lot of UV and the effects should be apparent in a relatively short period of time.

The necessary lamp spectra, design of the test chamber and acceleration factors are detailed in the standard ASTM G154 - see https://www.astm.org/g0154-23.html .  The 311nm lamps are often the most practical.  Those sell for horrendous prices if purchased as materials testing lamps - but the same phosphor is used in the better quality reptile lamps mixed with daylight 965 triphosphors.  By using larger quantities of those or at shorter distance to raise the irradiance to the same levels as the lamps without white light, the same effects can be attained.

James, could you look at this (somewhat related) thread,  https://www.lighting-gallery.net/index.php?topic=15687