Not to be an alarmist, but--in the photo, it looks like the two incisors on either side of the 'growing in' adult incisors *might* be in the way(for want of a better term)of the downward growth progress of the two 'growing in' incisors--see the angle of the inside wall of those teeth? If it were me, I believe I'd have my equine vet take a look....anything that impedes or is 'in the way of' a tooth trying to grow in, might affect its proper growth. Not saying this is the case here, but I'd have it looked at....FWIW.(In my experience, teeth come in at different rates in different horses; don't know that there's a hard-and-fast answer to your question, in that respect.)
Best wishes,
Margo