Well..I don't know for sure. I usually didn't get really sick (like not being able to prevent throwing up) until about 5-ish weeks. But that's not saying I didn't feel slightly nauseous in the weeks prior, it's just that it was so weak I didn't pay much attention.
Conner was the only one I really tried for...and didn't succeed for a few months. I got a bit crazy during that period and would make myself sick thinking about it. So much so that when I actually did get pregnant, I didn't think that was it at all. I had felt mildly nauseous for a while (can't remember now if it was 1-3 weeks?) before I actually got the morning sickness, but I remember being surprised that this was it
So my short answer is it's impossible to tell. Even with Conner being my 7th pregnancy (I've had 3 losses), I couldn't tell I was pregnant till I was late & tested.