Interesting thread, but I'm guessing it takes 8-12 months because a given shop is juggling multiple projects.
I just want to see that work is progressing on the car. If so I don't mind waiting however long it takes. I would hate to feel like someone is rushing to finish just to get it done. What I don't like is when my job gets pushed aside because some *** hole is demanding his car be done by a certain date, and all the patient customers have to wait for his car to be finished.
To any 1 customer at at any time, there's only 1 car in your whole shop...theirs. The only ones that get full every week attention are the ones that stay current. Your guy is late/slow pay? Simple math here, progress is equal to payment. I have said to customers "...then come and get your ****in car. Pay me up and take it." Any shop that does multiple jobs is obligated to every customer equally and the customer is obligated to pay in a timely fashion. That's an issue that will jade every stroke of the sandpaper when it gets ****ed up. I haven't lost 1 yet that I didn't choose to toss but it does ****. Damn bro, been there once before. Stand your ground and get all "black and white" with the project and the customer. You're right so get your way. Don't internalize what an uninitiated customer tosses out at you. Don't fight or argue, DON'T RE-NEGOTIATE! If you do you were wrong. Try to help the dude regain his enthusiasm. Again, good luck.
How many time has this guy been late on his payments? Add up the weeks and tell him you were late so many week on payments so your car is going to be so many weeks late. By the sounds of it he will be getting it early.
When he comes back in one week give him his car as is...done or not..I don't really think it's worth the aggrevation of putting up with his arrogant demanding ********..and by now you've got most of the money and job done. Tell him to come over and let him use a DA for 8 hours and see how he likes it and how much he gets done. Let him piss and moan to someone else.