In my 40+ years of raising poultry it is a individual and breed specific condition on how long they stay broody.
I have one bantam that is broody 10 months of the year, she is a OEG bantam that is constantly broody. She will hatch every egg I put under her but the one problem with her is that she will tend to the chicks for about two weeks then go back to sitting.
I have had white leghorns that will show broodiness for a week or two then go back to being a leghorn.
When I end up with too many that are broody I try to break them. Here is what I do with with the standard breeds (it is not so successful on the bantam breeds but can work). I have a wire cage that I hang on the side of the coop and place the broodies in with water and food. Do not give them anything to sit in or on. It generally take about a week to snap them out of it.