choose a project- start it- if you run out of fabric for it you need to get more
if you can get it all done then you had enough
a persons stash is kind of (relative) many of us have extensive stashes- and can create/make quilts for years with it- others need to (suppliment) you only know when you are working on projects- if you choose a pattern and start pulling fabrics out of your stash for the pattern and do not have what you need---you need to do some shopping- or improvise- but there is no way to really know for sure until you start pulling fab's out for the project you are going to work on. my french braid quilt was made completely from stash fabrics- in fact i've been working on stash quilts for a about 4 years now- i do on occassion have to shop- for a coordinate-or blender- or a backing- but 95% of my quilts are created from fabrics i have on hand.