It would depend on the kind of quilts you like to do. I don't stash solid colors (unless VERY good price - something that just can not be beat), I have found that you can always find them. For me FQ are fine for scraps or FQ blocks. I try to get between 1/2 and a yard (more again if $$ is good or it is just some thing that I LOVE), for my stash. I have found that you can always put different things with it. Good luck. When it comes to a STASH - it is always personal to each quilter and this is some thing that will change as YOUR quilting changes too.
Happy Sewing