When you set up a drive, you can specify how many hours of use before cleaning is done. This is done when you add the drive under devices - There is a box that says "Cleaning Frequency". This only works for drives in Robotics units. If you insert a cleaning tape manually, the clean count is not changed (Such as in stand-alone drives) so to keeo it current, you would insert it manually then run tpclean -M, which will reset the count to 0.

You should also check with your vendor about how often the drives should be cleaned.  We were told to only clean ours when the drive tells us that it needs to be cleaned, as the cleaning process does cause some wear on your drive heads.

here is document telling how to use netbackup cleaning utility and setting clean frequency.

http://seer.entsupport.symantec.com/docs/242905.htm

... the SL500 has a "Configurable cleaning cartridge slot for [automatic] tape drive cleaning by library".

That way the drives will only be cleaned when they need to be cleaned & therefore will lead to less wear & tear on your tape drive heads.

Definitely do not OVER clean your drives.

If you library and drive firmware support it, you can use the Tape Alerts and NBU will automatically clean the drives when they report that they need to be cleaned.  Basically the same thing as Andy's suggestion, just having NBU do it instead of the library.  Plus, if you let NBU do it, you can have as many cleaning tapes in as many slots as you like ;)