There really isn't a "norm", but often 2 weeks for a show in trouble. Sometimes there is a month, particularly when close to a common closing time (The end of summer, right after the New Years influx). When a show is long running, you might get a closing date further out.
With Comet? I dont' think anything will be normal. I HAVE tix for the 2nd performance after Oak's departure, and I don't know if I'll get to see it. Again.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.