You will likely start to see some preflower signs by week four. They make a quick stretch for a week or two, then really start to fill out. Same flow as photos but at a highspeed - I love that part.
In my somewhat limited experience (currently on my fourth tent grow of autos) they are pretty consistently ready in the 75-85 day window, depending on personal preference.
Photone app for your phone will give you a rough estimate of light measurements. You have to pay to get ppfd readings, but it’s cheap.
I’m running a grow journal of my current auto grow if you want to check it out. Here’s a pic, with the largest plants at day 37 or so, to give you a sense of pacing.