How to charge iphone 5 correctly

You can and should charge as often and as long as possible... deep discharges are hard on lithium batteries. Normal mobile use will result in the one charge cycle/month that Apple recommends. Also, you cannot overcharge the device, thus you can leave it plugged in as long as you like.

http://www.apple.com/batteries/iphone.html

Note: a charge cycle equals one 100% discharge/recharge, two 50% discharge/recharge, four 25% discharge/recharge, etc. Also, sometimes you may need recalibrate the battery meter by fully discharging (until it shuts off) and then fully recharging (don't unplug until 100%).

The purpose of the full cycle charge is to recalibrate the battery gauge. Lithium chemistry batteries are notoriously hard to manage, because their voltage does not drop linearly with state of charge, as most other secondary cells do. So the battery gauge monitors battery charge going in and battery drain going out, and computes the remaining life from this. But the battery's capacity drops with age, and the calculation isn't perfect. So after a while the gauge no longer knows what the state of charge actually is. The most obvious symptom of this is when the phone suddenly shuts off well above 1% remaining. When you run the battery all the way down, then charge fully, the charging circuit in the phone can re-establish the end points.

Anyway, the answer to your question is charge it whenever you want, except that you should periodically run it all the way down, then fully charge it (once a month, as Apple suggests). But not every day. According to Apple this does not affect the total lifetime of the battery.

Oh, one other point. The iPhone will not let you deep discharge the battery. When it shuts off at 1% there is still significant charge left. It's unusable charge, except to keep iOS alive. If you fly a plane there are 2 numbers you need to know - total fuel capacity, and useable fuel capacity, which is always lower. Same with Lithium batteries in electronics.