@Jay *****
different offices have different "under review" periods believe it or not
Chiang Mai's is 15 WORKING days <- which can vary depending on when you apply, if there are holidays etc.
Although up there they normally tell people you need a minimum of 21 days left on your current stamp to apply π
It really doesn't have anything to do them being busy or not, and AFAIK only Chiang Mai has that wacky 15 working day processing.
Most all the other offices run under the "you need a minimum of 15 days left on your stamp" to apply for an in country non-immigrant type visa