the best timing of mating.

Question
my chocolate lab went into heat 10 days ago tried to mate her with male dog but she just snarled at him.  when would it be best to try her again she is 3 years old

Answer
Hopefully both dogs have their minimum health clearances.

10 days from the start of heat is typically too soon, and the bitch snarling at him is also telling you the same thing.  But if the 2 dogs live together, she may never let him mate no matter how ready she is.  

In the US many serious breeders are having progesterone testing done at the vets office to determine the best day to breed.  Many will opt to do a side by side artificial insemination to avoid the nastiness that can happen if the bitch doesn't want to mate.  

If that's not possible, try it again in about 4-7 days and see what happens.