Hi! Best way to tell when to harvest is to grab a powerful magnifier and look at the trichomes. (60X is good, you can get little microscopes that have lights on them which is what I use.)
An immature trichome will look seethrough, like water.
A mature (and full of thc) trichome will look 'milky'
And a trichome that's passed that stage will start to look amber.
The more amber you have the more 'couch lock' your weed will feel like. Most people want as many milky as possible with few amber.
I personally like that 'couch lock' feel so I prefer a few amber trichs, say 10-20%
Here's a link to a pic that shows the stages:
https://i.redd.it/gc2r0v1qnzk31.png