How frequently should one update a wallet.dat file backup?
The Bitcoin-Qt/bitcoind client's keypool has by default 100 Bitcoin addresses. Each spend transaction that has change chews up one of those addresses. Additionally each click on New Address will consume an address from the keypool as well. So you should be able to get by without a new backup until after 100 spend transactions plus any requests for a New Address.
So if you do 10 spend transactions a month, and receive funds 5 times a month you would be fine with a backup every 4 months maybe.
A backup plan should consider that backups fail, so multiple backups don't hurt anything.
The size of the keypool can be configured so that frequent use can occur without having to make backups as frequently. For instance, weekly backups would be fine for a wallet that does 100 transactions a week but the keypool has size of 250.
When the wallet is encrypted, no keys are added to the pool until an action that requires the passphrase to be entered. Backup strategy should take this into account.
And finally, if you import a private key, that key will not exist in any previously made backups.
Technically a single backup is sufficient. The bitcoin network has a record of all your coins and transactions. The wallet simply stores your private key and wallet address.
If you generate additional bitcoin addresses or import additional private keys then you may need to create a new backup.
With that said you can never have too many backups and I always recommend backing up any file before making changes especially when dealing with money.