The itchiness, for me, came about two weeks in. Just let it go, and after about a week it goes away.
The best advice I’ve heard is to let it grow completely for four to six weeks, even if you aren’t looking for a full beard. That way you don’t shave off more than you would like to accidentally.
Blonde beards grow faster than darker beards, by the way. And good luck to all you redheads!