In general, I think it takes a heifer a little longer to cycle after having a calf - thus one of the many reasons we breed our heifers to calve 30 days before the rest of the cows.
Like the PPS have said 45 days is about the earliest. When we were trying to synch recips, the ET guys said don't even bother with cows less than 60 days out because the success rate just isnt' there.