
If the bitch is staying with the stud then I would mate every day over three days, but if you have to travel each time then you leave a day between, that's the only real reason for it.
You increase your chances of success by mating over a longer period, three or four days rather than two, though your best chances are to mate the bitch until she won't stand, this shows she has gone over.
If a bitch is mated at the optimum time she is unlikely to stand for longer than 3 days, but if she is mated a bit early she will stand longer, up to 5 - 7 days.
Some bitches will stand from just before ovulation, other won't until ovulation or after. The eggs take about two days to be ready for fertilisation anyway, but then canine sperm should live for at least 4 days, but be in best condition for two, and can survive a week.
Some males will only mate a bitch when she is at optimum, no matter what she says ;) Older bitches that have had litters before and have high libido will often be happy to stand for extended periods, not good for predicting the best time at all ;)
Add to that the bitches who ovulate very early eg day three, or those that are very late eg day 23 it makes for and interesting time trying to get yoru bitch pregnant.