Anywhere between 5-12 days is probably the most typical dryhopping time frame for commercial & homebrewers alike. Some people will go outside these bounds and it doesn't make their processes wrong. Additionally, some people will complain about grassy flavors and automatically attribute that to increased length of dryhop. But this is not true in all cases for every brewer. There could be other factors that come into play here.
I too would recommend going by taste. Use a sanitized turkey baster to take a very quick sample, cover with airlock, and compare the dryhopped beer at 5 days, then 8, and then 12. Whatever anyone tells you, the beer will not be ruined either way. But at least you'll know the dryhop length that you personally prefer in the future.