How much semen will the average vagina hold?

It depends on the size of the vagina, but since it is elastic it can hold an estimated amount of 45-70 mL.

The average vagina can hold a lot more than any human male penis can deliver. Plus it does not stay in there very long. A portion of it mixes with the vaginal fluids and is expelled shortly after intercourse as the vagina contracts to its regular size and shape. The rest of it is absorbed by the female reproductive tract. The actual sperm, however have a different destination. They swim upsteam through the fallopian tubes and if they don't participate in fertilization make their way into the abdomen to die and get absorbed in the female's body.