As an LQS employee, I can't tell you how many times I've had a customer return to the shop in search of more of a fabric they "just bought last week"; but, ALAS! it is all gone!
My suggestion: if you see it and love it, buy as much as you can afford RIGHT NOW! There are no guarantees that it will be here when you return.
My boss orders fabric months before it arrives, and most of the time cannot order more later. Fabric manufacturers make only as many bolts as are ordered. Now, there are certain fabrics that are considered "classic", such as Moda Marbles, Kona solids, etc. but even then dye lots are different and colors can be slightly off from bolt to bolt.
The bottom line is, buy a little more than you think you need of any particular fabric.