With my first pg I ovulated on day 11 so that is when I "concieved" but I did not have sex after day 6 (yes straight after af finished!), so I in another sence I "conceived" on day 6!!! But my cycles were only 26 days long at that time.
It depends on the length of your menstral cycle and the length of your luteal phase (time between ovulation and period). Even on a standard 28 day cycle you could ovulate day 18 or day 12 or anywhere in between.
So considering that sperm can live upto 5 days, the earliest you could "concieve" is possibly around day 7 on a standard 28 day cycle.
Obviously if you have a longer or shorter cycle than usual you can concieve earlier or later.....
Sorry this is long winded but I hope it helps answer your question.