Be careful what you wish for. Stash can take over and if you don't have the space and you don't like scrappy quilts then it can become a hinderance. That said, when I first started, I always bought a little extra for the projects that I was actually working on. Then I started buying fabric "just because." DH thought for the longest time that fabric HAD to be bought in full yard increments, starting with 1 yard. lol.
I have now become "full up" in my stash containers and have to be extremely frugal (self-imposed).
Some people make a swatch book of their stash and take that with them to the fabric store so they can match and supplement. Others just buy what suits their fancy.
Anything from 1 yard of a panel size to 5 1/2 yards for a potential backing (or more). Trust me, if you want to grow your stash - every Fat Quarter helps.