If you are using scraps it is hard to know if you will have enough of one pattern...so I save my scraps for a very scrappy quilt.
If I have some amount of fabric I estimate how many time I want to use it in a block...Say I am doing a 9 patch and I want all the centers to be the same...and I am doing 4" squares...and I will need 16 blocks. I measure my fabric to see if I can get 16 4" squares out of it. Hope that helps