Usually if you are just backing up your "software" drive, the interval can be pretty long since most of that is static anyway. If you do have to set the way back machine pretty far back, you may see a lot of updates coming down but you seldom actually lose much. I will kick off a manual backup if I added some significant software change and it has run enough to see that it is OK.
OTOH the image from a week or so ago has saved me several times when a new package, or even an "update" put me in the ditch.
