There is no magic number, no magic method. There are only one magic join buttons.
10 seconds is long enough for a surfer to spot shit. Which is why the number of views is meaningless.
2 minutes is long enough for a guy to realise this scene is poor. Or the same as he has seen over and over and over again. Just with a different girl on a different sofa doing the same old shit.
10 minutes is long enough for the freeloader to get off and move on with his day. Or for a customer to think "This is fantastic and I have to see the HD version of lots of these."
As all we really have left to sell is the HD version. How many want to buy a HD version of mediocre porn?
Yes they may go pirate it.
Most stuff on Tubes isn't good enough to motivate an impulse or planned buy.
But I think I said this before once.