Yeah, we tend to go by 4-6 weeks on here for the starting side effects. Fluoxetine can take a couple of weeks longer though because of it's unique length and how it works at a certain stage on the brain. Otherwise, 4-6 weeks should be enough. However, that doesn't mean it is a slog all the way through that and whilst I struggled at the start on Citalopram, I found by week 3 it was decreasing. On Duloxetine it was much more intense but thankfully by day 11 it dropped off a lot.
SSRI's work in 2 stages. The first stage is that filling of the "space" with more Serotonin and then after about a month studies of some have shown there has been a decrease in the holes in the receptor sites where it would normally pass through. The latter is the real aim.
I found with Citalopram that it was 2-3 months to feel much. Some people find results faster and some even longer, it's so individual as ever. I think you have to pass through the side effect window first, some people seem to think they should be feeling better before then but that's unlikely.
If you go the 4-6 weeks and still have the side effects, then a chat with your doctor is worth it. They may advise a switch as some people find some meds just don't work for them or just give them side effects only.
I found the first thing that hit me was the nausea and dizziness side. This makes sense anyway because the majority of your Serotonin is in the gut, where you have many more receptors. My first med, Citalopram, has been seen to interact with a specific gut receptor that tied in with the nausea hence why they say take them with food which has been shown to reduce the effects. I was also incredibly hungry but couldn't actually eat.
So, some of the side effects are likely because your gut has to deal with it too. I found this passed really quickly though (about 2-3 days) and moved onto nausea, mood swings, etc.
It's certainly true that you will feel the side effects pretty quickly. If you had an allergic reaction, you would feel that far quicker than most side effects.
So, you haven't had a bad reaction, really you are doing very well, but once past the 4-6 weeks, these new issues should go.