Maybe never. What breed and is it from a hatchery or breeder?
The bulk of breeds are no longer setters but an individual of any breed can get the urge to raise a family. First time broodies don't always do a good job. I usually discourage first timers.
Orpingtons are known as broody breeds. Half of mine did set on eggs, the other half never did. Leghorns are not setters but I had a black one raise several broods for me. You just never know.
Here's 2 breed charts I like that list a breed's tendency to be setters.
http://www.albc-usa.org/documents/chickenbreedcomparison.pdf 8th column.
http://www.sagehenfarmlodi.com/chooks/chooks.html second last column
The presence of a rooster should make the eggs fertile. However, if a hen is prone to be a setter, she'll do so whether or not the eggs are fertile or even if there are any eggs at all in the nest.