There isn't really a set time. I had the same thing as you. was very confused as last af was one day on the 16th dec, and then we only had one 'episode' on the 21st dec. turns out, my dates are bang on for having concieved on the 21st dec, by the scan. I thought it was a bit od as this was only 5 days after af....????? So there you go, can happen anytime. You'll need a scan to get the exact dates.
Congrats & Good Luck
Linear Mode
