The bandwidth notification is sent out automatically when the bandwidth reaches 80%. The bandwidth is measured monthly, and resets on the 1st of the month.
If you get a notification earlier in the month you can calculate if extra bandwidth will be required. Extra bandwidth can be purchased if required
If you regularly find yourself requiring extra bandwidth, you also have the option of upgrading your account. This can be done at any time, there are no penalties for upgrading your account and the payment will be worked out pro-rata for your existing account.