Nice!
Your regime seems perfectly reasonable/safe to me. Some people have the Envy on for 8hrs at a time!
In the manual it states to:
-Give tubes 15 minutes to warm up after turning on (I give mine about 5 lol)
-Do not use for more than 8hrs at a time (I rarely reach 4 as my Envy is in the main family living area)
-Leave for 1hr to cool (this relates more to when your are swapping tubes).
I have read that the longevity of the tubes can also be related to the amount of times they are turned on/off (heated and then cooled). Idk really but I think tubes will last for many thousands of hours.
Just use it when you want to and enjoy it