If you continue to use the Bitcoin-Qt wallet after you back it up, you'll need to create a new backup on a regular basis.
With default settings, the Bitcoin-Qt wallet only backs up enough to recover the next 100 addresses used. Bitcoin-Qt uses a new address every time you click the "New Address" button, but it also uses an new address (which it keeps hidden from you) every time you send a transaction.
Woah what? Damn this shit is confusing. So if I backup once a week, is that enough?With default settings, the Bitcoin-Qt wallet only backs up enough to recover the next 100 addresses used. Bitcoin-Qt uses a new address every time you click the "New Address" button, but it also uses an new address (which it keeps hidden from you) every time you send a transaction.
That depends on how many addresses and transactions you are using per day. If you create a business that results in sending over 100 transactions per day, then a weekly backup won't be enough. If you generate 1 new address per week, and send 2 transactions per month, then backing up once a year will be enough.
Is it ok to backup over and delete the old backups or should I save them all?
Generally, I recommend keeping an approximate count of the number of times you've clicked the "New Address" button as well as the number of transactions that you've sent since the last backup. When the sum of those 2 numbers is approximately 25 or 30, it's a good time to create a new backup.
Then I recommend keeping the 3 most recent backups at all times. That way if the most recent backup is lost, damaged, or otherwise unusable you'll still have another backup or two that you can use that will have access to all your bitcoins.
