Simply seeding for the 1+ hour you are watching the movie is typically enough to keep a torrent swarm healthy now that the majority of users have 10 Mbps+upload speeds. In that time, you can easily upload more data than you downloaded, meaning you are a net benefit for the swarm.
In a general sense, any video content where the bitrate is lower than the typical users upload bitrate could work for a webtorrent only swarm of 'regular' users who aren't going to do anything special for network health.
Only if they seed more than they leech. It has little to do with time. If you take a year to seed more than you leeched, you're a positive contribution to the swarm.
Problem is: they can't watch movies in their browser because they can only leech from people who are seeding from a webtorrent client.
Let's say I have a movie I want to let other people stream with their browser. I am obligated to use some javascript horror webtorrent-compatible client. If I already have a seedbox running transmission I simply cannot use that for this purpose, which is a shame
In a general sense, any video content where the bitrate is lower than the typical users upload bitrate could work for a webtorrent only swarm of 'regular' users who aren't going to do anything special for network health.