
On average it is day 10, but like SharonM said, some bitches do ovulate earlier so perhaps start at day 5 to be sure.
Your vet should advise you once they have the results on how often she will need to be tested again, but I would make sure your vet can get the results within 5 hours, this has been a difficult task for myself, one vet surgery I use can't give me results for 24 hours, and if you are travelling a distance this can be very annoying as the timing maybe missed!
Some advice also, once she has had the test say in the morning, if you have not heard anything from your vets by about 4pm then keep ringing them to ask for the results before the labs close! the first pre mate test I did I left it for the vets to phone me like they said they would but heard nothing when I called them the next morning I was told that the results had been received last evening but no one had phoned me!! :( :(
Good luck !! xx