I would suggest that it be something like:
where represents the probability that a mouse rejects on day , is the probability the graft survives the trial and is probaility that it rejects on day given it rejects. Then assume given it rejects that has a normal distribution (remember that the recorded day is rounded to the appropriate integer).
Now you have two things to test:
1.That the probability that the grafts survive are equal
2.That the mean times to graft rejection given rejection are equal.