Normal temporary sources increase the background by a factor up to 4. Adding a factor of 2 for safety, we get x10 as maximum expected increase above dark sky level. The time scale of this gain variation is of few minutes, too short to be monitored by reference light sources. Therefore we should put a rather low limit on the maximum acceptable gain variation. We should probably study what is the time taken by the PMT to recover after the exposure.
Some information is provided by the manufacturer.
For the time being we require the half-life of the PMTs to be not less than 15 years.