Variable timing Switch

I have a simple question.

I have 2 wires... and I want to switch on/off. But I also want to control the switch on/off time.

---------- (Switch) -------------
1st wire 2nd wire

I want to control the timing.
For example... I want to switch on/off in 1 ms, or 34 msec. So the timing be variable. 1 or 10 msec of precission.

How can I build that?

There are many ways to do that, and some methods depend on whether you are switching AC or DC. The amount of current you are switching is a factor to consider in any method. How you wish to control the switch is also a consideration, whether manually with a pushbutton, for example, or remotely with a trigger signal. Too many variables left out in your post, but you may want to search on time-delay relays. Also, it's not clear whether the switch turns on after the delay or off after the delay.