You should look at the SnapMirror transfer sizes, that should show you your change rate so you can estimate what your snapshots will consume. Viewing them from the snapmirror.log (accessible via the etc$ share on 7-mode or the 'spi' web interface for cDOT) will let you gather historical values. Keep in mind that certain processes (defragmentation, database reindexing, etc.) will cause large changes so make sure you account for any of these in the time period you use for estimation.
