How much I purchase varies widely. In the past few years my quilts have contained more fabrics than those made 20 years ago. Now its not uncommon for my quilts to have 10 or more fabrics.. so the amount/yardage of a single fabric I purchase for "speculation" or stash as gone down to 1 yard. If its a really spectacular fabric that might be a border fabric between 2 1/2 or 3 yards. If its a great background in a White on white or cream on cream print and the price is the best I have seen in a long time I would buy 6 yards or maybe the whole bolt.
So much depends on your quilting style , or the type of quilts you like to make and how many quilts you might make in a year.
If I had any advise to a newcomer to quilting fun... only buy what you can afford , and there will always be fabrics you will fall in love with. There is the addiction of acquiring beautiful textiles.. and its starts with the first purchase.