Backup as often as you feel comfortable. If you honestly don't make any changes for weeks, simply backup when you do. A backup is just a safety net. If worst case scenario your iPad stopped working and had to be restored to factory settings...or even replaced, you can use your backup to restore everything. As long as you're happy with whatever your backup contains if you ever need it, that's all that counts. More here:
http://www.apple.com/icloud/
FWIW I'm now keeping a backup of my iOS devices on my computer (via iTunes) as well as one in (on?) iCloud. I won't be religious about the one on my computer...but if anything happened to the one in the "cloud" I'll still have something to fall back on. I have two backups (one on and one off site) for all of my computers as well. I'm a bit retentive that way...although it did save my bacon one time.