Sorry you're feeling like this CosmoGirl!
It can be any length of time at all. I know of a girl who is about 2/3 weeks away from her due date and still occasionally gets bad days. Not what you wanted to hear, I'm sure, but in reality it's what can happen.
I found out I was pregnant by visiting the doctors as I thought I had a kidney infection - lower back and side ache, constant nausea, etc - and found out I was pregnant! I had an inkling I was, but thought I must have an illness on top of that because of how bad I was feeling. He did give me antibiotics that were safe to take, just incase I did have some sort of infection, but nope, the 24/7 nausea carried on up until around 18 weeks.
When I say 24/7 I mean there really was no relief other than for the actual few seconds/minutes when I was being sick (probably around once a day, most days) but it was the nausea more than anything. I took 3 weeks sick from work and some annual leave too. I could of done with more but couldn't afford it. You feel like you're in this sickness haze and you just find your own way through it.
Foods will be trial and error. I went back to the doctors because I was living off Mcvities cheese flavour biscuits, and volvic water lemon and lime flavour, and was convinced I had Hyperemesis. They weren't too concerned though as I was holding down fluids, and in the early days that's the main thing. So like someone else said, fill yourself up with water or whatever fluids you can hold down, and foods such as biscuits, dry toast, bread, even cereal without the milk.
I was recommended Pregnacare tablets by someone and was told they give you some sort of relief but unfortunately not for me. I did carry on taking them for 12 weeks though for the baby's benefit.
Wishing you the best of luck 