Depends.... B29s, for exemple, are 95% of time above 25k when they do a strats run.
Most of veteran bombers pilots fly at 20 k or more to avoid being insta-overrun by uppers when they reach enemy af area.
Exception for the TU2, which seems to be very capable of defending itself, as its very maneuverable (compared to other bombers) and can, if needed, try to engage the attacking interceptor (if there are less than 3 ofc).
By using smart RPM you can spare a lot of fuel, and, if your plane allows it, drop tanks are the solution.
The only plane that wont last very long after climbing to 20k+ is the LA-7, but this alt isnt the best for it anyway....
As others wrote in this thread, you got many different solutions to last longer in your plane, you should try these solutions, i'm sure it will solve your fuel concerns...

L.
There is no point in climbing to 20k, and if you do you will find yourself diving down to 13k at least to get a kill. This is one part that makes this game unique, gotta conserve and watch out for your ammo and gas,, that unlimited crap is for am amateurs... It is a part with being generally new to this game.. it is very difficult. That is why you gotta be aggressive and not be afraid to go to the DA to fight or to ask people to help with ACM because in this game it is important.
+1