You can base it off of what you see with your bare eyes. I've noticed that once trichomes get cloudy/milky, you can tell just by looking at them with your naked eye.
You can also use the method some growers use, which is watching the hairs develop. Some people say that once 4/5 hairs are mature, the plant is good to harvest.
I recommend getting a 30x-60x jeweler's loupe, so you can keep tabs on trichomes and properly diagnose when your plant is going to give you the medicinal properties you're after. they're only around -.
Good luck!