It really depends on what kind of quilter you are. Very project oriented, one at a time quilters may not have a huge stash; they might buy as they need for each project. If I really like a fabric I buy 2 or 2.5 yards, if see a fabric and my brain lights up with "that'll work perfect with fabrics a, b, c in my stash, I get 1 - 2 yards.
When you decide to build your stash, the first thing you MUST do is to decide how you will store/organize them.
Stash becomes overwhelming in a very short time! For each piece of fabric you purchase, I recommend that you measure, fold, mark the length and width on the fabric selvage, and then store it**. Since it's so early in your stash building career, you could, if you wanted to, keep a spreadsheet *of your fabrics for future reference.
*Note that I have not done that.
** I waited until I was 7 years in on building stash to organize. Started almost a year ago and am about 80% done now.
Only because i can afford to buy fabric (Thank you, God!), I have a fairly extensive stash. It's loosely divided into these categories:
1) Blenders - it's amazing how the blenders in my stash can make a quilt work. About 1/3 of my stash is blenders. I love that I don't always have to run out to get a blender when starting a quilt.
2) Christmas - almost always bought on sale. I've been collecting it for over 7 years and I am finally making a Christmas quilt for my grandson. I love Christmas fabrics, but the theme is limiting, so they aren't prolifically used. The more general Christmas fabrics are used in not-Christmas quilts, though, and often make a pretty backing.
3) Fabrics I love so I bought them. I hate to say it, but most of these are on an invisible "look but don't cut" list in my head. Lately I've used a few, but it's very hard to cut them. Generally, I use them in a big block quilt so I can still see lots of the fabric.
4) Extreme sale fabrics - I don't love this fabric, but it's 60 cents a yard so how can I pass it by? Some of these have managed to become 'best buys' because they were perfect for a child's quilt, made a great backing, or added a bit of whimsy, funk, etc. to a quilt. A few have actually 'made' the quilt! These fabrics are mostly good quality, just not stuff that I particularly like all by themselves.
5) All other fabrics - what can I say? I love fabric, to see it, touch it, fold it, create with it.
6) Precuts - I like them, have enough of them to last me awhile without being overwhelmed by them, and use them when the spirit moves me to. I don't buy too many of these. I do like the jelly rolls that are made up of solid colors like white, black, dark brown or red, because they make instant sashing.
7) Kits - I have about 6 undone kits. I've had most of them for at least 2 years and not touched one of them.
Good luck and happy hunting!