Of course the answer to this question is that you can never buy too much fabric! However there are normally two issues that may hold you back: (1) storage space and (2) budget! Some people don’t like buying fabric if they don’t have a project in mind, but I tend to use the fabric to inspire the project!
I’ve spent the weekend organizing my new office space. A big part of this is categorizing and organizing my fabric. Yes, I have a lot of fabric! But much of it comes from my first year of quilting, when I was primarily making fused wall hangings. I would see a fabric I liked and buy a fat quarter. Occasionally I would buy half a yard. Fast forward four years and I have a large — and very odd — collection of fat quarters. My interests have transitioned to making traditional quilts, so I don’t have nearly as much need for novelty fabric. I also usually need more than 1/4 yard.
I also had a lot of fabric from a buying spree at a discount store. I was a brand new quilter and couldn’t resist the cheap prices. Many of my purchases were a name brand, but it was a low-end version that looked good and felt very stiff. Today, I would never use that quality of fabric in my quilts, but at the time I didn’t recognize the differences in quality.
One thing I have learned is that you don’t need to buy every fabric in a line. I have many collections where I purchased 1/2 yard of at least a half-dozen fabrics. It’s not enough fabric for anything more than a lap quilt. Now I buy the focus fabric and maybe one other. You can always find blenders, and it keeps your quilts from looking like an advertisement for a particular designer.
I have now transitioned to buying 2 or 3 yards of the focus fabric. (I want at least enough that I can cut the borders without have to piece them.) And then I buy the blenders when I figure out exactly what I’ll be making.