As for knowing how long your battery will last, like minnflyer said you can probably get at least 3 flights out of a 600 mah battery (I'm guessing that's what size you have), but if you don't want it to be guess work.

you can get a expanded scale voltmeter or a voltwatch, the esv is more accurate but you have to plug it into your plane to read it. The voltwatch is always on but it's not nearly as accurate and takes some skill to read.

esv
http://www3.towerhobbies.com/cgi-bin...?&I=LXNK81&P=7

voltwatch
http://www3.towerhobbies.com/cgi-bin...?&I=LXHDJ2&P=7

Austin