I like the advice the others gave about the trichomes. The problem is that for people that have standard setups, 250-400 watt HPS etc the plants take longer than the advertised time; at least in my experience. Perhaps it's a marketing tactic or if they actually do finish in 2 months they are using 1000 plus watts of lighting, CO2 tanks and everything dialed in to perfection when testing the grow time.
The danger is harvesting too early, the buds may look really good but if the tricome development is immature the high is shit. My first soil plant was looking really good to me at the time and was pretty large so I harvested it. I barley felt any kind of a buzz even after smoking a cigar sized blunt. It needed much more time, what a waste, a shoe box jammed pack full of dried bud that was completely bunk.
Here's some stuff I harvested in the past month that turned out good. (400 watt HPS) Most were advertised 60-70 days.
Kabala - 89-92 days
Auto #1 - 75 days
Auto Jack Herer - approx 125 days (was stunted but turned out killer)
Critical Jack - 90 days
After some experience you can almost get a good idea when they are almost finished visually. Most or all of the hairs will be red/orange and receded, not sticking out. On the tip of the bud if there are still white hairs standing up it needs a bit more time. The leaves will begin to yellow starting at the tips and continue to yellow.
When you figure out the pics it would help to post them.