Counting Characters and Alerting when close to limit

I've created a variable in the hierarchy view called myAlert2

I have added this code to the change event of yellowDescription:

if(yellowDescription.rawValue.toString().length > 1050 && myAlert2 == "NOT"){

     xfa.host.messageBox("You may enter about 50 more characters in the Description field.");

     xfa.host.setFocus(this);

     //xfa.host.messageBox(alert);

     myAlert2 = "DONE";

}

This code works, but will provide a messageBox on each keystroke after the condition is met.  I would like to have the alert only come up once.