I took for 6 months, 20mg of fluoxetine, which did help me out a bit.
Took about 2 months, maybe a couple weeks more, before I could say they were really helping. Felt a bit weird before that, and sort of robot-like for a bit after, which was useful.
I only stopped as I had to leave the country for a month with work, and stupidly didn't go to the doctor beforehand for another prescription.
It was pretty clear that the dose would have needed to be upped even a few months in. I guess you build up a tolerance quickly...
Depends how you are feeling, but I was struggling to even walk to work at points, due to just feeling so negative about the place I live.
List of side effects as long as your arm on fluoxetine, but I think it's more of a concern at higher doses.
Placebo effect is a good crutch for the first few weeks, "I'm taking something, so I know I'll be ok".