I always think time limits are good, especially when punishments like electro are in the offing. You start to see the panic begin when the sub feels time is running out, the Dom giving time checks every now and then but no clock visible. It adds an edge to the scene, the first shock to the balls coming as a surprise and the sweat begins. What was a relatively "simple" although not painless quest becomes a race to finish with the sub knowing the shocks could come at any point. 