The time flooding in SWMM 5 is pretty simple:
1. It is the sum of all time steps in which excess flow is coming out of the top of the manhole or
2. If you are using the ponding option then it is the time the surface pond has more than zero depth.
via Blogger http://www.swmm5.net/2013/08/how-is-time-of-flooding-calculated-in.html