Anyway, I was just wondering if I could keep my address forever (as in for the rest of my life), like a bank account.
Also how often should I back up my bitcoin-qt wallet?
Quote
The wallet contains a pool of queued keys. By default there are 100 keys in the key pool. The size of the pool is configurable using the "-keypool" command line argument. When you need an address for whatever reason (send, “new address”, generation, etc.), the key is not actually generated freshly, but taken from this pool. A brand new address is generated to fill the pool back to 100. So when a backup is first created, it has all of your old keys plus 100 unused keys. After sending a transaction, it has 99 unused keys. After a total of 100 new-key actions, you will start using keys that are not in your backup. Since the backup does not have the private keys necessary for authorizing spends of these coins, restoring from the old backup will cause you to lose Bitcoins.
I found this on the Bitcoin.it wiki. Confused me greatly.