You are correct there is nothing that specifically defines an optimal length of time. I have found that for most presentations 5 min or less is enough to create a therapeutic effect and tends to be what I use most often.
Here is a link to a nice article written by Frank and Dave.
Here are two examples of ENS for low back pain I recently shared on instagram.
This was a chronic issue where she has had L5 distribution paint for several months and continued nerve tension. ENS done < 5 min
This was a friend of mine who’s back tightened up the day prior more of an acute locked feeling that I suspect was discogenic in nature. <5 min
Here is a published case study for low back pain where they applied for 20 minutes.