How much of a fabric do you buy
 

when you are buying for your stash?

2-3 yards if I really like a focus fabric.

Yardage I buy 3 yards of each design I like. If I buy precuts, I've learned to buy at least 6 if charm packs, 2 if layer cakes and 2 if jelly rolls. For backing fabric I buy at least seven yards, enough for a queen size quilt.

I will buy 3 to 4 yards sometimes....mostly, I buy too much and my stash just keeps on growing.

Two yards if I really like it. Three if I love love love it.

Although I haven't bought specifically "for stash" yet, I have bought additional prints in a line "to use with this project if I need it." I like having options.
Having said that, I recently added a couple more fabrics for a Blooming Nine Patch project- 1 yard each, and "while I was in there" (Etsy) I added "a-couple-of-yards-of-this-coordinating-print-just-in-case-I-decide-I-like-it-better-than-one-of-the-others-or-maybe-I-won't-use-it-at-all-on-this-project-but-I-like-it"

That's essentially my stash-building mantra at the moment.

If I like it, I get one yard. If I love it, 2 yards. If I really love it, 3 yards. Lately, seems like love it and really love it have been winning out. Pre cuts, 2.5 inch strips if roll has 40 strips I buy one. Less than 40 strips I buy 2. Charms I buy three. I haven't bought any layer cakes yet, but I think one would be enough.

Depends on how mudh I like it. Probably 2 - 4 yards.

I would do 3 because a lot of patters call for over 2yds so you don't want to be short. I would get more if you envision using it in borders as well like 4-5yds