By default, the wallet creates 100 keys as a buffer (not sure if it's a rolling buffer, you'd have to ask Gavin or someone).
Currently it's rolling, but a pull request is in that would change it to (by default) top you off to 100 addresses only when you go below 75 unused addresses. We'll see if that goes anywhere.
So you have 100 different addresses to pick from with a fresh wallet backup before you need to backup again.
Be sure to see Dude65535's note above.
So yes, use only one address and you'll never need to backup again.
Not entirely true. (Almost) every time you send bitcoins, a new address from the pool is used to return your "change". As such, the easiest way right now to be safe is to just back up every 100 transactions.