I'm 43 and have made many rag quilts that measure 7 x 9 blocks. Usually 8" batting and 10" fabric (prior to snipping). If I'm doing nothing else I can make one in 24 hours. Even my hands will feel the pain after that and I have rag snips, bought from JoAnns. Seeing how I do count sleep time in that...take away ~8 hours...or 9 if we subtract eating and bathroom trips. 15 hours is a lot of time, if you're charging what a normal person should charge. She may just change her mind. Anyways, if you do launder these at home, clean out the lint trap once every 5 minutes. I'm NOT kidding. Only one at a time! We've got high efficiency washer and dryer.
I wouldn't recommend the pre-cut squares, then you have to worry that you're not catching the snipped part in your sewing. Careful to not snip too far. If you do, then make sure you fix it by sewing around the too far snipped area immediately. If not after it's dried, you will see a big 'flppt' poking out into the smooth side of the quilt.
Good luck!