When I go to about this mac to look at my storage space is says that 31 gigs is being used up for "backups" on my 128 GB air that's also 1/4th of the storage space! What is this backup use referring to?
Local snapshots are periodically condensed into daily or weekly snapshots to minimize the space used on your disk. If your disk is low on space, Time Machine stops creating new snapshots, and some or all existing snapshots may be removed to make space available for applications to use. If sufficient disk space becomes available again, Time Machine resumes creating local snapshots. This means your disk will have the same amount of available space as it would if Time Machine were not enabled.
You do realize it just does the previous two weeks, right?
Thus the max usage ought to be what you've changed in the past 14 days. It'll spike a bunch on a new system due to all the "new" stuff you're copying over, but seems like it ought to level out and reduce aftter you get past the first few weeks with the new system, no?
Wouldn't know yet, only had my system for 1 week ;-).