I'm not sure how long it's supposed to take, but for me, I had IB for 4 days. I actually had thought that I was starting my period. The first day it was a little heavier and I used a tampon because I thought that I was starting (oops, such a no-no if you're pg). It ended up that it was pretty much just spotting, but it did last for 4 days.
I did have spotting just before and including on my temp drop. Since you have a longer LP than I did, I would say that it is still a possibility that it was IB. Good luck and can't wait to see tomorrow's temps
Here is my pregnancy chart if you'd like to look at it: