For the derivation of pressure change in momentum is divided by time taken by particle in covering the distance and not time of collision? It is understood that time of collision cannot be taken. But why time of particle traveling is taken.

Pressure = Force/Area

Pressure = (change in momentum/time)Area

 

So, if you will carefully look, the time component that comes into play when the momentum of the object is being changed. So, it depends on which pressure are you talking about. Pressure at the time of collision should consider the time during collision, while the pressure defined otherwise, would depend on the time taken to cover a certain distance.