Got my approval packet via email today. Originally started my application July 8th, but didn't complete it because I didn't see Tesla on the approved dealer list. Learned that they got approved in mid-September and completed my application Sept 24th. So I would have to guess that they take applications based on the date started, not necessarily when it was completed.
EDIT: I also received this text from Tesla a couple of days ago, so the timing appears to be perfect:
Tesla Update - We are reaching out regarding your Model Y order, RNXXXXXXXX, which is currently still on hold.
Please reply OCTOBER if you would like to take delivery this month and a Tesla representative will reach out to discuss next steps. We'll keep your order on hold until you reach out, so please reply if you would like to take delivery soon.