I rarely make small wall-size quilts and don't care much for scrappies, so I never buy the precuts - charm packs or layer cakes, etc. I rarely buy fat quarters. If I think a fabric would make a good focus for a quilt, I would buy 2 or 3 or 4 yards. If I think it would make a good accent fabric, maybe a yard or two. If I am thinking the back for a quilt, I might get 5 or 6 yards. If I have no idea what I would use it for, but I just love it, I would probably get 2 yards.
Like Kathy Osterby said above, it all depends on your style and your plans. The more you quilt, the better you will get at knowing your style!
BTW - welcome!