That’s up to you. I need at least 4-6 inches extra on two opposing sides so the quilt can be loaded correctly. If the back is too small, issues can crop up while the quilt “shrinks in on itself” during the quilting process. More is better!
I provide all threads and have a vast inventory. If there is a specialty thread you would like let's talk about what would look best in your quilt. I typically only use Glide or So Fine! because they are low lint and look great in the quilt.
Turnaround time depends on the season. Christmas and Grad/Wedding seasons are busier so plan ahead for those. Rush orders can be requested for an additional charge. Normally I'm about three to four months out.