My daughter took the shots for 7-1/2 years.  Occasionally we would be late for the shot - usually only 1-3 days.  We tried to do them every 28 days, but sometimes it just didn't work out that way.  I think we were as much as a week late once or twice and it did not have any adverse effects on her.  Of course, to be most effective, you do want to keep as close to the 28 day cycle as you can.  How old is your daughter and how long has she been on the shots?