I agree with Marty. Thing is if you are to really consider those that are less into the hobby and maybe don't factor in things like maintenance, then you have an actual date. These fly by night people have tried grabbing their trains that need fixing, brought it to shops or called our fellow mechanics here on the forum and find out that it isn't going to get fixed because they're busy as heck. I have been told by my local train store as well as a few folks that they call in late November or even a couple of days before Christmas. No engine is going to get worked on and turned around that late in the game.
Good rule of thumb for maintenance and repairs is to inquire before summer ends. I'd even say contact whomever would be doing the work to see what sort of workload they may or may not have as well as planned vacations. I know I've avoided plenty of headaches that way.
Also, with September being here, local train stores should have increased business leading up to Christmas(and possibly after) before everything winds down. I know mine will get crazy busy even with flash sales and sidewalk sales(before it gets too cold).