How do you know when to harvest your goods?

So, il be doing my first ever grow with a friend starting this week.

I was thinking around 3-5 months, but when do you know its time to harvest your goods?

I'l also be doing this outside if it makes any difference

https://www.rollitup.org/harvesting-curing/28072-harvest-time-tutorial.html
thats got pics and a step by step guide on when and how to harvest then dry and then cure your plants.

thanks mate