Really easy to harvest to early. Really hard to harvest to late.
Be as patient as you can. The last couple of week are when a lot of the bulking up happens, making your patience worth it.
Some will advise going by the color of the trichomes, some will just look at the plant as a whole.
For viewing the trichomes I recommend getting a digital microscope that connects to your phone. They come wired or wireless. Amazon. Reasonably inexpensive.
You can take this quality of photo:
For anyone just starting out, I have been recommending this site:
Growing weed can be easy, though some marijuana grow tutorials make it seem like you need a degree in horticulture. Learn how to plant a seed & start growing with as little time and effort as possible, or read our advanced tutorials for monstrous yields!
www.growweedeasy.com
I like the way it is set up in a tutorial format.
Good luck with your upcoming harvest!
WillieP