^^^ Typical dwell times on the stringbed are 4-5 ms according to Yandell, Cross and others. The dwell time for a ground interaction (a bounce) is typically 3-4 ms according to Rod Cross, I believe. It might
be possible to increase stringbed dwell time with a touch/drop volley however, for normal strokes, the dwell time will probably not vary much at all (10% or so) regardless of how fast one swings.
The dwell time can be increased to 7 ms or so by significantly reducing stringbed stiffness
(which includes frame stiffness). Reducing string tension is one way to do this.