You have to look at the trichomes, those little things on the plant that look like "mushrooms"
When they are all milky (cloudy) and some are amber, it's the perfect time to harvest
To look at the trichomes, you need a microscope of at least 60x (magnification)
You can buy it at your grow shop, on Amazon, or on Aliexpress
It will be very useful in the future if you keep growing