5 minutes. You can find information on the frequencies available for Browser Monitors here.
If you are looking to have shorter frequencies, you can do this by adding in more locations, so if you wanted to check your site every minute, you could test it from 5 locations, each at a 5 minute interval. We spread the executions out evenly, so that would result in an execution being run against you location every minute.
Synthetic SME and community advocate.
