Films aren't constrained in length technically, but they all end up about the same length - probably related to how long one can sit passively for. What really

Films aren't constrained in length technically, but they all end up about the same length - probably related to how long one can sit passively for. What really drives the 30/60 minute timeframe? Is it medium or culture? Does shifting online really change that? A medium can change quite quickly, but culture changes far slower.