You can charge it at any level that suits you needs
But I read that completely draining the battery at least once a week and then fully charging it keeps it fresh
Sent from my iPhone 5s Gold 64GB
Goodness, aren't we past the complete battery drain days? I thought completely draining the newer Li-On batteries was totally unnecessary anymore, and harmful at worst.
From PC World:
"The tricks for maximizing your lithium-ion battery's useful lifespan—that is, the number of times you can recharge it before it no longer accepts a charge—are pretty basic. Three things will prematurely age a lithium-ion battery: consistently draining it to the automatic shutdown point, exposing it to heat, and overcharging/overvoltage-charging. That last practice is actually dangerous and can lead to fire or even an explosion."
And check out this Battery University article for further indepth analysis.
They think the sweet spot is to drain to 40% and then recharge.