
I have had a maiden bitch accept a maiden dog days 12, 14, 15, 16 and 18. In my breed and my bitches I have usually had fertile matings at days 14 to 17. I have usually had bitches willing over about 5 days.
Technically speaking you would expect a bitch to accept a dog from ovulation to four days after as by then the ova are dying off. The ova are not fertilizable until about 48 hours after ovulation, so accepting a dog more than two days before ovulation would be pointless as the amount of sperm in good condition will be minimal after about 4 days.