It usually takes 1-3 days, but it will all leak out eventually. Once you cum inside of a female dog the sperm immediately separates from the ejaculate. The sperm is desperately trying to find some eggs to impregnate, and will be sorely disappointed! Lol
The ejaculate will be flushed out by her urinating. The sperm on the other hand is normally absorbed by the mucus lining of the uterus/vaginal tract once it dies, then it's processed, and then urinated out of the body as waste. Also, don't give an enema to a female canine, it'll likely just throw off her natural pH balance, leaving her vulnerable to infections.