You have to trade off the situation.
Running a GaAs junction hot for a long time will accelerate failure, probably due to metal migration.
Turning the FET on and off VERY frequently will temperature cycle the part to the extreme, and cause some sort of microcracking eventually.
If you only care about Heat, how about turning it part way off. Like go from 12 watts to 1 watt. That will keep the temperature extremes smaller, and allow more temperature cycles.
Of course, if you only transmit for 100 mS in every 10 minutes, I would turn it completely off.