I was able to get into seiza at about four months.
As John mentions, it is more the ability to get into the position and rise from it. At first (and still when I do it at home), I put a bench in front to help get down an up. Obviously now it's not a problem.
I have found it (as I'm sure you know) a great way to stretch the muscles on the thighs and shins, etc. No limitations for me, I'm now back to good workouts (bag, weights, full stretching), and have taken up yoga again.
It takes time, though - I did not begin any martial arts workout until one year or so, yoga at about 14 months (I was not a practitioner before, for others who are, they came back to it earlier). The main thing is to heal for the first 6 months to a year, the rest of your life comes back based on a good foundation of healing.