Believe it or not, I heard a rumour that one airline's SOP was to select reverse for a fixed number of seconds, which was what prompted me to post the question to you guys and gals!
I suggest that statement aptly matches your name
Reverse thrust is also defined by the amount of thrust. If you use too much thrust below 60kts then the eflux will crawl forward of the engine inlet and lift stuff off the runway for the engine to run into.
Time is not a parameter in its use.