You want to make sure that you give it a full 8 hour charge occasionally as well. You can't over charge the iPhone, and this helps with battery calibration. 😀
Yeah, as Julien said, charging the battery until it appears full on-screen takes about two hours. It's only really about 85% full at that point though and there is another 15% / 20% or so that you can still squeeze in there on top of that which takes quite a bit longer. A lot of manufacturers do this apparently to make it seem like the battery charges to full quickly, as all Lithium-Ion batteries behave in this way.