If you are using C# and are using properties, you can just capture a DateTime inside the property setter, or as part of the property changed event. Then every time it's set just see if the last set was before a certain time. If it is, do something.

If you are not using C#, this (data encapsulation) is just one of the great reasons you should consider the switch ;-). Seriously, if you are not and are using UnityScript you will need to encapsulate the access of that data inside of a method (like a setter) and do what I described above. Just exposing a public variable will not do this.

The last and really nasty, not elegant solution would be to note the value in a second variable, then periodically, perhaps in Update or LateUpdate see if the original is different note the time.

if (variable != shadowVariable)
{
   //is the timespan since the last noted change less than my 
   //predefined period.
   if (currentTime - lastChangedTime < predefinedTimePeriod)
   {
      DoThing();   
   }
   shadowVariable = variable;
   lastChangedTime = currentTime;
}

That's psuedocode of course, but hopefully you get the idea.