At this point it shouldn't be a thing anymore. We hit a point where a lot of people are YouTubers now. So a lot of the times its tough for them to make time even though they actually like your vids. Ex. Its tough for me to watch peoples content regardless of length I watch a lot of the peoples on this forums vids but I can't ever find the time to comment because I either gotta record, or do something else. I can barely watch anime nowadays. lol
as a percentage I'd say roughly as follows:
0-35% is a fail
36-45% is average
61-70ish% Best possible
71% or over can't really be achieved as you will always get micro views that leave immediately that drag your score down.
I aim for 60% but that's because my audience is really small right now. But based on the length of your videos i'd aim for 50% at-least. Most people can spare a min or two and watch a 3 min video. If you want to extend your watch time consider adding more content to your video besides form and exercise technique such as the benefits of doing that exercise, specially what muscle groups the exercise targets, and common mistakes to avoid.