
Many people would not show a bitch again at all after mating. I would say you would be OK under 3 weeks after mating, after that anything crossing the placenta could harm the pups. In fact in canine pregnancy according to the book of the bitch the most risky time is is 3 1/2 to 5 weeks in whelp.
My bitch had her litter nearly 10 weeks ago, and if she were smooth coated I would think I could show her in another two weeks as she is now fit and her boobs have gone, just got big nipples left.
In fact I will be very lucky if she will be fit to show by Midland Counties, and probably not until LKA as she needs to grow her coat back, as the whole of a bitches coat is lost about 8 weeks after whelping.