Typically, you can trust a battery to last about one season, flying twice a week total 8-9 flights weekly. Whatever performance you get beyond that is a bonus. We usually lose batteries in the off-season due to not cycling them, so you should invest in a cycle charger or alteast charge and discharge your NiCd or NiMh batteries every two-three weeks. While I do like the idea of keeping a watch on the battery, its drifting from the point. For a glow plane, all you need to be sure of is that the battery is in condition good enough to perform when required. Put a voltwatch on the airplane (paste it to the fuselage side) and make sure it doesnt go to yellow with all sticks deflected to their max extents. Stop flying and recharge as soon as it goes into yellow. Think that is about enough for a glow airplane in the beginning. Also get yourself a 5cell (6V) NiMh atleast 1200mah or more for safety sake. You can go on to a LiPo or LiFe or that sort later when you want to make things more complex. Till then enjoy your trainer

Ameyam