What happens if I schedule a Map/Reduce script to run every 2 hours, but it takes >2 hours? Will the next deployment just wait until the first one is completed? or does the next deployment get "cancelled"?

should be deferred and then run

Similar to scheduled script types, they should also run as unique instances. So I believe the next instance will start while the old one continues. I might be wrong here as this is an edge case. Confirm this with NS support.