I started feeling it the same day, but most people say to give it about three weeks. Some people take as long as two months to start feeling medications work. I had an overall calm and happy feeling come over me out of nowhere a couple of hours after taking it, so I am pretty sure it had to be from the Prozac. By the way, I take a generic version, not the name brand.
I didn't seem to have any side effects at all at first. After a few days I started to feel the drowsiness from it, but it hasn't gone away after a few months. It isn't severe though, so I don't plan on stopping it. I am usually tired anyway!
The trickiest side effect that I developed a couple of weeks into taking Prozac was acid reflux. I noticed that for some reason I had a hard time keeping the pill down. I would drink a lot of water and would feel it stuck in my throat still. I believe it would sit in my throat for too long and cause throat irritation, heartburn, and the acid reflux. It was difficult to exercise without vomiting a little bit and it was uncomfortable to eat fruits and vegetables. I started turning my head to the side as I take the pill and it helps it go down, or I wrap it in some bread. The plastic capsule seems to get stuck for some reason, even though I don't have this problem with any other pill. Now I take Prozac at night instead of morning. No more acid reflux! Taking it at night gives it some time before I start eating in the morning, so the food doesn't bother me. It still works well and probably helps with the drowsiness.
As for my weight, I am happy to say that Prozac has not caused any gain! This was my main worry as I try very hard to stay in shape. I work out hard and eat a good amount and Prozac hasn't affected me, so I am thrilled about that.
Good luck and I hope it works well for you!