Have not flowered auto yet personally, I'd let someone experienced with it give you reply to your question. You will be basing harvest time on trichomes color more so then anything, but as mentioned autos are a little different pace then photos, although I'm sure you still will beable to use 10x or 50-60x magnifying to see color of trichomes.
I'm sure in general plants will bulk up in end weeks of flowering, but autos are autos and under limited lighting probably won't form massive colas like photos vegged for 2 months under MH/HPS, what lighting are you using btw just curious what you grew that auto with.