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

I seldom buy fq's as I can get a half yard of fabric for not much more and I would have 2 fq's If I wished to cut it.

I've been told if you love the fabric get a least 2yds. I built a stash but mostly around prints and if I were to do it over, I'd buy solids, tone on tones, batiks, then purchase additional for individual projects. FQ's work great for small projects. I almost never buy 1/4yds as if off grain I lose a lot squaring up. I buy 1/2yds when not sure what fabric is for, unless it could be focus fabric/border then 2 1/2 to 3yds.

That's also how I do it, no less than 1/2 yard. For me, fat quarters are a tease. You pull them out of your stash and they will be perfect for your project. But there just isn't enough of it to do much with. So you are disappointed you didn't purchase more when you had the chance. Avoid the disappointment and buy more to start with.

Fat quarters are really good for appliquers who use little pieces of this and that. Piecers, who rotary cut, usually prefer to have the full 40" width.

I started off by buying fat quarters but now I regret it as there's never quite enough fabric for what I want to do. I now buy at least 2 metres; 3 if I think the fabric will be used for borders.

When I was building my stash, I bought 2 yards of all I liked and 3 of what I really loved - up to 5 yards. I haven't been sorry that I did that and have more than enough fabric to last me the rest of my life. I still can't resist buying fabric, but restrict it to 1 yard cuts now. Fat Quarters just don't work well for me - way too small to do much with and I have more than enough scraps from the larger cut fabrics.

Six years ago, when I first started my spiral into the addiction of collecting fabric errr.. quilting, I'd buy a half yard here and there for stash.
I quickly amended that practice to buying a yard.
Now.. I'll still get a yard, but if it's a great sale or something I really like, I'll buy anywhere from 2 yards to the rest of the bolt.

I am relatively a newbie, too, and I find I am leaning toward the 1 yd unless it is something more special. I tried a fat quarters pack (just to see what they were) but since my interests are so far varing since I am newly retired, to quilting, sewing dolls, trying different things, I am leaning toward the 1 yd. Besides the yard in many instances seems to be cheaper than the FQ. Don't flog me - I said I was new to this, and bless this board for all of its knowledge and wisdom. :-)

Marysewfun

The mistakes I have made with a stash- purchasing too much of something that was so-so, and not buying enough of my oh-my-gosh-I-can't-live-without-it fabric!

I agree with buying smaller amounts of the fabric that you "like", more of what you "love" and the whole bolt or as much as you can afford of that "can't-live-without-it" fabric. So rate each fabric by how much you like it before you deciding how much you need. When you have a pattern already in mind, and then it's how much extra you are purchasing past that pattern amount for your stash.

If I were starting a stash, then I would be routinely searching fabric sales, 2nds from Connecting Threads, etc, and plan to do that over a year's time- I would not plan to build it in one sitting, because you won't find everything that you like now.