Welcome from the Bay Area of California. I'm with most of the rest of the folks here - about 3-4 yards if I love it.
I usually make lap quilts, and my husband and son are both over 6 feet tall. So any 4 yard pieces could be used for backing on a quilt. Because of budget, I am TRYING to not buy material except at sales. I have found old clothing and even old sheets can be good sources of material. THERE ARE NO QUILT POLICE HERE TO TELL YOU WHAT YOU HAVE TO DO!
I would ask yourself what your main focus will be on quilting.
For myself, I do scrappy community quilts. These don't need large pieces of material. If you are into heirloom quality quilts, then you want larger pieces so you can make that quilt sing in the same colors throughout.
Talk to people. You will find sources of material from relatives, neighbors, church members and others once you speak up about your craft. Believe us, you will have a stash in NO TIME! Good luck and let us know how you do.