If it's from the same cut as my Purple Gorilla strain you have about 4 weeks.
But things are never so cut and dry!
It's more of a harvest 'window'. Some plants have a short harvest window, say less then a week. While this plant seems to have a longer harvest window starting from the last week of September to the middle of October.
I'd let it go for as long as it keeps swelling and only pick it if too many tricomes start to go amber.
hope that helps
Porky