I start as soon as I know the plant is female.
I'm also a huge fan of collecting and properly storing pollen for later. This makes 'seeding' your female autoflowers as simple as picking a packet from the freezer, waiting for it to warm up to room temp, and then pollinating your female. Quite a bit easier than trying to 'time' a male or reversed female to dump pollen at the right time.
It does mean a slight delay in making seeds though.