I just put it on the Dock when i get home, so it gets charged every day usually. But its rarely more than 30% burnt when I charge it, so I think I could go for 2 days.
I certainly do use my iPhone, though perhaps not as much as I would like to use it. But when you have a full-time job, it makes it hard to be on the phone too much.
I have to charge it every night, otherwise (as other people have also posted) it dies during the second day. I use it a lot every day, so this doesn't surprise or bother me.
As for battery replacement, I fully expect to be on my next iPhone long before battery performance becomes an issue. I'm estimating that it'll be at least 2 years before my battery drops below 80% efficiency (which is still enough for my current usage habits), so I've got plenty of time. I don't expect that I'll be upgrading very soon as I'm comfortable with 8 gigs and EDGE, and almost everything else is software upgradeable.
Charge Cycles
A properly maintained iPhone battery is designed to retain up to 80% of its original capacity at 400 full charge and discharge cycles. You may choose to replace your battery when it no longer holds sufficient charge to meet your needs.
This link seems to answer the question on the best way to charge LI batteries: