how many hold time take a relay to switch on and off?

 

Hold time? If you talking about the operate and release time, it depends upon the size of the relay. It's usually in the tens of milliseconds range. If you look at the datasheet for a specific relay, it will tell you that information.

 

Depends on the armature inertia of the relay.
Max.

 

how many hold time take a relay to switch on and off?

If you need a fast release time, a simple back emf protection diode across the coil won't do, when the field collapses producing a back emf - which is shunted by the diode, that causes current to continue to flow in the coil longer than would happen without the protection diode. This delays the release.

You need 2 emf protection diodes - one of them a zener, with the zener oriented to clamp the back emf, its forward voltage will prevent coil energising, so you have to put an ordinary diode in inverse series to prevent that problem.