Odd Alpha Tween Question?

1 
 this.onEnterFrame=function(){
 if(nav._alpha<100){
 nav._alpha=nav._alpha+=5;
 if(nav._alpha>99){
 delete this.onEnterFrame;
 gotoAndPlay(66);
 }
 }
 }

How many frames will it take to go from 0-100? Is it 20? Is that how the script works?