Some guys say mid October for the first frost which would stunt or stop growth. Give them 60-90 days of growth for optimum, but like I said, the weather is a crap shoot anyway. What I am thinking this year? The sooner the better. We had mowing 5-7weeks early. We had dry weather early and long. It is mid August and we are seeing the resurgence in lawns like we would in late August to early Sept. They took off like mad last week. Not saying it means anything, but the way the rest of the year has gone, an early frost or temperature drop wouldn't surprise me.
We got terrible tonnage off ours last year. Then we realized we should have put up some seclusion fences. The deer ate them to the ground! Turns out the turnips were still growing well. Pulled some of them out and they were baseball to softball sized!