Thats a bit tricky, as it varies among models i think. My good DX30 has an optimum a bit over room temperature, works best when it has handheld for a bit and/or powered up for half a minute... i'd say between 30 and 35 degrees case temperature. When it's left on for a long time (several minutes at least) it dims a bit as it gets over 35.
My dodgy, but now rewired and working again DX20 is best at around 25 and dims a bit when it heats to 35.
Btw, these optimum values are for the green output, the pump diode will put out more power when colder, but either due to pump wavelength mismatch or KTP (in)efficiency, the overal optimum works out as described.