ok, the answer here is, charge it as often as you want.
the phone's battery life is 300-400 charge cycles. a cycle being the same as 100% use of the battery. so say you use 40% of it the first day and chaged it, then 60% the next day and charged it, the two charges over the two days would equal one cycle. If you use on average of 1 charge ever 2 days then you'll battery life would easily last over 2 years. if you can get one cycle every 3 days then you could get 3 to years out of your battery life.