Hi Spring in my experience it really varies.
I believe I have been pregnant four times all together, and twice I knew within maybe 2 days of conception, the first time I just knew - it was far too early for a BFP, and by the time I could've confirmed it I had a massive bleed, and obviously wasn't pg anymore!
Second time around I had really bad symptoms from almost the moment of conception, but didn't realise what the symptoms actually meant until AF was late - Drs told me there was NO WAY i was pregnant, and were treating me for inner ear problems instead - My daughter is now 3 and certainly NOT and Ear infection!
Third time around same symptoms as with DD started almost immediately PO, and I KNEW what it meant that time - had to wait for 15 days for BFP, and shortly after lost the baby in a car crash.
Am now pg again, and this time around I hadn't got a clue, only did a test because I didn't want the sudden shock of AF arriving after all my mates fell pregnant and I was gutted it wasn't me, so really big shock to get several BFPs! according to my dates I am now just shy of 11 weks, and can't wait to get my scan now to convince me I am not imgining this one because it is so different to the others!
Sorry to ramble, hope its of some help.
Take care
Cxx