
On average most bitches are unlike to be receptive/fertile for more than five days.
Some bitches will stand from about a day before ovulation, but fertilisation cannot occur until about two days later after the ova have undergone division, and these then remain viable fro roughly 2 days. The eggs are released over about 1 or two days.
So you can see that the optimum time for sperm to meet eggs is from two to four days after ovulation, so if you hit the right time two matings two days apart should cover all bases,a nd in fact some dogs and bitches will not mate outside this time.
A bitch that is mated early to a very fertile stud could still conceive if mated five days before ovulation as sperm can survive up to a week, but the chances are obviously reduced.
It is also pointless mating a bitch after the eggs have died at more than four days from ovulation.
It would seem most people err on the side of mating too early.
Personally I am quite Happy if my bitch is staying with the stud to let her mate for up to five days if the first mating seems to be early, but otherwise over a three day period, especially when the male has said no not yet, and the bitch is saying yes.
As for worrying about different puppy ages, the fertilised eggs all implant about the same time and size of pups is more to do with relative position and efficiency of placentas than a difference in age.