Most likely her entire life.

Senior dogs can and do get pregnant. Fertility will slow down as the dog ages, but even a 14 year old dog can become pregnant.

It's best to spay your dog, she'll not only be healthier, she'll live longer, and you can avoid any unwanted pups. :)

Generally for the rest of their lives.
We had a client recently where we had to do an emergency c-section on their TWELVE YEAR OLD Boxer, because they didn't think she could get pregnant...