I ran my GG#4 for 13 weeks...turned out great!
Just as a general rule, in my experience, there are very few (if any) photoperiod strains that are truly ripe at 8 weeks. Ten weeks is a minimum starting point for ripening and sometimes up to 15 weeks and beyond, if it's a phenotype that leans toward the Sativa side of the coin. I don't grow autos at all, so I can't comment on them.