BUT how often you start using a new private key depends on the wallet you use.
As one example:
The Bitcoin-Qt wallet uses a new private key every time it gets change back from a transaction (which is pretty much every transaction). It has a pool of 100 keys that is uses for this. When you back up your Bitcoin-Qt wallet.dat file it backs up the next 100 keys that will be used for this purpose.
So, you should back it up before you use all those keys.
I back up my Bitcoin-Qt wallet every dozen or so transactions or every time I generate a new key by pressing the "new key" button or every time I import a new private key.
Other wallets have differing requirements.
damn
I use Bitcoin-Qt also
So I need to back it up every 100 transactions
good thing I stopped using faucets and stuff = less transactions