The sadest reflection here is the number of postings who seem unaware that most modern a/c actually use 3 gyro driven instruments - 2 vacuum driven (Ai/DI typically) and one ELECTRICALLY driven (TI/TC).....
To be honest if you a fully qualifed PPL and dont know the answer to which instruments are driven what way then whoever checked you out (CRI/FI) was SERIOUSLY amiss....this info can save you life (depending on the nature of your flying).
Re: Batteries....
anywhere from 30mins-1hr depending on loading with the master on...it is often stated in the POH/FM - again if you havent looked here *first* - or worse still didnt think of looking here first then again you have had duff instruction....
The *best* idea is to have a bloody clue how to navigate instead of w**king over the GPS and failing to look out of the aircraft...
P.S. Compasses dont take electrical power. I can fly a straight line without *ANY* electrics - and indeed proved this recently when I really did have an alternator failure in flight....Maps & a proper Plan dont require power either. What do you do if the GPS unlocks or fails due to bad battery?!?
Pen & Paper...the ONLY way to safely navigate....