Validity days of the URL's Partners share for their users to use.
For how many days are my Partners' URL's valid?
The unique Partner URL's are valid for as long as you decide.
Valid means that if one of your Partner's users clicks on the URL and it takes them to a registration or a sale that you have determined in your Program, the time between the initial click and the following action (Registration or Sale) is the validity time of that URL. In that sense, the commission will be paid to the Partner that shares that URL in the case of it remaining valid.
How to change the validity time of your URLs?
All the URLs managed by your Partners in your Programs will have the same validity time, which will be defined by you. To change it, go to your Profile > Settings > Payments.
On the Payments tab you will find four options, the last of these is 'Cookies tracking time'. That option refers to the validity time, as you'll see in the Help tooltip.
By default, 30 days or a month is the validity time that you can modify.