Good morning bran! Would you have access to a jewelers loupe or a good magnifying glass? There are two really good ways to tell when an auto is done; the first is by checking the trichs, the second is by looking at your leaves.
If you can get some sort of magnifier (a camera with a good macro setting can even capture them,) you can look at the trichomes on your leaves/buds to see what state they are in.
They typically come in three phases:
Clear heads (literally you can see through them,) this is considered by most to NOT be ready yet. You can harvest with clear trichs but I think you're going to be sorely disappointed and the high will be MUCH different compared to if you had let the trichs go further.
Cloudy/Milky heads (they look like a milky swirl in them, can't see through them.) This is what I think most people consider nearing "done" with their plants. Nearing peak THC production and little degradation has happened.
Amber heads (reddish/brownish/amber colored hues in the trich heads.) These are the heads that typically give cannabis a more narcotic "couch lock" feel. Once you start ambering, past that you start to see degradation of THC as it start converting into other things.
Look around for yourself, but I think you'll find a lot of people try to harvest with a certain ratio of amber to milky trichs (like 1:4 amber to milky or something to that effect.)
If you can, I would really suggest trying to take sample buds during each phase of the trich to see what kind of high you like best out of the strain. They really do make a difference too!
If you don't have access to a magnifer, check out this thread created by Muddy, one of AFN's senior members and an incredibly knowledgeable guy:
Sticky: When to harvest autos? Your leaves will tell you.
Here's another good read:
Sticky: The life cycle of auto flowering cannabis.
Hope that helps a bit bro! :karma Cloud: