The only way to know for sure is to invest in a scope of some sort. Depending on the stone you are after, you want the trichomes to either be just turning cloudy, just turning amber, or 50% of the trichomes are amber. The more amber, the deeper, heavier more couch lock stone you get. Also, the older the plant is, the more THC that is converted to CBD.
If you are using non-organic fertilizer, don't forget you have to flush for a week so you have to plan for that as well. If you are using soil or all organics, no need to flush, although I do anyway just to be sure.
Finally, it depends on the genetics. Indica's tend to finish fast, while some sativa's can go 12 weeks. Do as much reading on your strain as you can. Good luck.