Can't really provide a guide since the time is a function of how much current your onboard electronics use. If you have a 300inoz strg servo, you'll get a lot less time than if you're only running a 80inzo servo.

Just watch how your RC reacts after you've run several tanks. Its pretty clear when the servos start slowing down telling you the pac voltage has dropped off and its time to recharge.

Any added time from lipo rx pac will come from increased lipo capacty (mah) since these are not high drain applications. You get increased run time with lipo over NiMh when NiMh is working hard enough to get hot (heat is wasted energy) due to the amp draw and NiMh IR. Since that doesn't happen with lipo, that wasted energy translates to extra run time with lipo.