There are several methods, but the easiest way is to have a interval variable and a timer variable. Lets say we want to trigger something every second:

var interval:float = 1; //for 1 second
var timer:float;

now:

function Start()
{
  timer = Time.time + interval; //the timer is equal to the time in the future
}
function Update()
{
  if(Time.time >= timer) //if the current time elapsed is equal to or greater than the timer
  {
    //do something
    timer = Time.time + interval; //set the timer again
  }

}

again, there are many ways to do it but this is a quick and dirty way I use often.