Adding to what mfutty said, another variable here is your network bandwith, or hardware setup of your backup gear to the amount of data that will need to be backed up. Presently we're running a 100Mb network and my Exchange server has so many mailboxes to backup that it takes close to 26hours to perform one full backup of all mailboxes at the brick level. This can cut into time of other jobs without proper planning i.e. redundant servers, backup jobs etc... Planning is everything when a restore determines whether or not you'll be with the company tomorrow.

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Systems Administrator
A+, Net+, i-Net+, CIW-A
MCP/2K, MCP+I, MCSE/NT4