It really depends what your style is. I know that is vague.
But, if your into applique, FQ's would work. If your into small/baby quilts or lap quilts, then you might want only 1-2 yds. If your into bigger quilts 3+yds might be what to buy.
Then there is the like, love, & OMG must have feelings you get with certain fabrics. That sometimes helps me to decide how much to purchase.
And another thing to take into consideration, do you have room for stash? If space is limited then only purchase what you have room for. IMHO to purchase and hide the fabric in tubs (never to be seen again), then you forget what you have. :? :-(