input_on_change.js

InputOnChange = Class.create({
  initialize: function(elementToWatch, functionToExecute, delay){
     this.id = null;
     this.functionToExecute = functionToExecute;
     this.delay = delay;
     elementToWatch.observe('keyup', this.onKeyUp.bindAsEventListener(this));
  },
  onKeyUp: function(event){
    if(this.id){
      window.clearTimeout(this.id);
    }
    this.id = this.functionToExecute.delay(this.delay, event);
  }
});

new InputOnChange($('myinput'), myajaxfunction, 3);