With autos. You really really do need a loop mate. You risk the chances of overripe and it'll be harsh as anything on your throat. Runied 3 of my harvests this round by pushing them to fast.
You'll notice a massive drop off in fluid uptake though. But, really can't be used in as a guide neither. 35-50 days it takes for an auto once it hits pre flower.
I do advise you get a phonescope or loop they're like 8$ from my growstore, phonescope.
Guesstimate 14-21 days