Learn English – Counting days to a particular event

The word you're looking for is since.

Almost ten days have passed since Apple’s WWDC 2015 event.

I disagree with Ekn.

"For the first few days" implies that something was going on continuously for days. While technically you probably thought about other things during those first few days, like eating and going to the bathroom, it is metaphorically valid, and certainly better in a literary since, to say that it was continuously ongoing. I would absolutely vote in favor of using "for" here.

However, "in" is not invalid. It says to me that at any time in those first few days that you attempted to figure it out, you couldn't, but not necessarily that you did so a lot for those first few days.

So both are valid, but in the context of a breakup I would use "for".