Dont worry about it, imo.
You can get great progress from never being in the gym longer than an hour, and ive heard of (not actually seen) research that says going too long has diminishing returns, even possibly to the point of going backwards. On the other hand, alot of well resepected programs have you in there for hours.
My point is, dont get fixated on a certain time you should be in the gym. Worry about the other stuff and it will work itself out.
I will say though, if you feel you are there too long, dont start skimping on the warmup.