You did well to get an 8 cell at day 2! It would depend as well on how fast they progress (so many variables!) At Charlie's cycle, I had two embies transferred at Day 5, but one was far more advanced than the other. They say that the embryo reaches the uterus at day 5 (at blast stage) so anytime after that, implantation can occur. Mostly it's 7-10dpo.

How much spotting are we talking about? And was it pink or brown? (sorry if TMI guys!)