So, you want to control the amount of time the user spends playing the game? (You sound like a parent to me!
In that case, you keep track of the time s/he has been playing -- could use Elapsed Time Express VI, for this. Check it out.
Yes it is that I want to control the amount of time the user spends playing the game (subjects are children). But I want too to program the game in state of pause like we can do it with wave sound in Labview (the function SO pause). It would be practical if I have non need to stop the game and restart it when a new response is produced.
The subject doesn't have to press the key on the keyboard (i.e., a keystroke), but you can simulate a keystroke from within your program to pause the Game. So, the first task is to find out if the Game can be paused at all. Next, is how it can be paused.
Most games have this functionality.. for instance, Doom can be paused with a "p" (but that may not be suitable for children)