As a minimum, most businesses should make nightly backups, using new tapes each night. Never use your tapes two nights in a row - this only doubles your chances for disaster. You can rotate your tapes so that the tape you used this Monday will be used next Monday, this Tuesday’s tape will again be used next Tuesday, and so on.
Your further backup needs are heavily dependent on your business. These needs should be analyzed and agreed upon by your software vendor(s) and the management personnel at your business. One key question must be "How much data can we afford to lose?". This one question alone may help you determine the proper backup strategy for your site.
Another question should include "For business purposes, how long must I keep my data electronically?". The special needs of your business may require that you make special tape backups each week, or each month.
For instance, suppose that you need to keep your financial information for each month of business operation for up to seven years for tax purposes. Many software applications allow you to keep this information on the computer system for more than one month, but if you need the data three years from now, it will either be (a) printed on paper, (b) stored on a tape, or (c) non-existent. Most businesses prefer both (a) and (b). This requirement demands that you make monthly backups of your system, and requires that you keep those backups in a safe (i.e., fire-proof) location.