6-8 weeks is a bit of a guide as well; depending on dosage, your metabolism etc it could be a bit longer (took about 3 months to really make a difference for me the first set).
Also, not all antidepressants work as well for everyone. I started on citalopram, which was ok when things were pretty low stress but weren't effective when I had to go back to uni (at least until they upped the dosage but then I had really bad side effects), then took sertraline for a while which did NOTHING for me, and now I'm taking venlafaxine. So...yea. It's hard to say.
But don't go changing prescription until after 6 months, realistically. If after 3 you don't seem to be having much effect, speak to your GP; it's likely they'll adjust your dose. If after 6+ inc a dosage change there is still no effect discuss the possibility of changing to a different kind.
It's more of an art than a science in figuring out what works, on both sides of the proverbial bedside.