Back log ( the amount of time it will take to do the projects scheduled ahead of your) time is differerent for each LongArm person.
I always ask before I send or drop off what the back log is. I ask when they want the quilt top . Once they get it typically no more than 3 weeks.
They should not be accepting your top if there is a long back log. Yes put you on the project list .. but no need to have the top "in house" till they are close to working on it. Most will call or e- mail that is time for you to get the top to them.
IF it is a highly custom piece I will send a photo so they can start to get ideas of how to best quilt the top.
I have had a "space" reserved for October with one Long Arm pro , since March.