Once the battery is fully charged, charging stops though, and the phone will just run on the mains power. You can safely leave an iPhone plugged in all night long, no worries. I've been doing that for years (I use a dock on the bedside table), and that way, it is always fully charged when you are ready to go in the morning.
In terms of battey use, obviously it varies drastcially as peoples pattern of use varies so much. But, with any smart phone if you are using the features a lot (email, especially push, texting, calling, web surfing, watching video and live feeds, reading news sites, ebooks, taking notes, using GPS/location services) etc, etc) you will need to charge the phone daily. If you are a heavy user, get yourself a car charger and an extra one for your office or work - I basically plug my iPhone in whenever it is convienent to do so (office, car, home, wherever I am easily near a place to plug it in) so it is always juiced up for when I am on the go.
Li polymer batteries are not harmed at all by very frequent partial discharge/charge cycles - in fact, they will last longest that way (it is repeated deep cycles that take a toll).