Microsoft Advertising runs your ads indefinitely unless you set end dates for specific ad groups. You can see and change your ad group’s end date by going to the ad group’s Settings and clicking Schedule under the Advanced settings section. When you select an end date, your ad will run until 11:59 PM on that date, based on the time zone that the account is set in. Learn more
You can set your ads to end by a certain date, which is particularly convenient when running ads for a special occasion, like a holiday sale (in such a case, you may also want to set a start date).
You can always edit an ad group's end date — before or after it passes — to make it stop running sooner or to make it start running again.