In my limited experience, that magic number has always been seven. The first few have a sense of newness to them which distracts...the next few bring a sense of understanding as you realize just what you've gotten yourself into...and by day seven you're finally able to come to terms with the new reality of the situation.
As we reach the seventh day of school, it still seems like the first day...but it's starting to move faster and with greater ease. By the end of the week, we'll hit the 5% mark and have our first sense of accomplishment of the year. Hopefully you all feel the same way as this long year finishes beginning to begin.