I would get it all at one time. I sewed all the blocks first which used the fabric up real fast then I could take the squares with me to join and make the windows.
You have to decide how you want it to look. If you want a uniform look buy all at once. If you don't care if there is a variation and don't want to buy it all at the same time then get just a little and hope there's more of a similar color later.