Counting time after an if condition, then doing a while until certain time is reached

I'm using C# within Visual Studio.

I'm trying to start counting time when an if condition is fulfilled, then do something within a while loop while the counter still hasn't reached a certain time.

To illustrate what I'm trying to do:

if(condition)
{
  //start counting time

  while(time < 5) 
  {
    //perform action
  }
}

The code is a bit more complicated, but I would know how to apply it from that example. Any help would be greatly appreciated!