Rich, I was mid-reply when yours went up.
I'll post anyway; a bit more detail...

Presuming that the slug will be traveling at the vapor flowing velocity, the 0,0 point of the force-time data would correspond to the leading edge of the slug arriving at the near-weld line of a bend. The rise time would correspond to the amount of time required for the slug to fully envelope the bend, that is, to travel from the near- to the far-weld line of the bend.
Time = Distance/Velocity
Distance = 1/4 of the arc-length of the circle corresponding to the bend radius.
If the pipe is 12" diameter, a long-radius bend radius = 18 inches, diameter of the corresponding circle = 36 inches.
In this case, rise time = 1/4*pi*36/Velocity.
Full load will be applied for as long as the slug is in the bend = slug length/velocity (which is added to the t=0 mark, not the rise time point).
As Rich says, the decay time will be the same as the rise time (same velocity, same distance, so same duration).