Time Machine Question

Is there a way to change the frequency of Time Machine Backups? Hourly is too frequent for my needs. I guess I'm wondering if the hourly spin-up spin down is going to reduce the lifespan of my external drive.

MY SITUATION
I have a Time Machine external drive hooked directly to my Mac Mini via FireWire, with my MacBook saving Time Machine backups via the wireless network. The Mini is entire a media server with relatively infrequent changes that need to be backed up. It's convenient to not have to think about backing up the Mini, but hourly seems like overkill.