cant edit full duration of text animation

Hi I want to decrees the duration of my text animation with this expression ,

I've tried edit the duration value but it dosen't work it seems like it change the scale or the overshot of the characters not the full duration 

how do i change the full duration of the animation in this expression ?

freq = 1;
decay = 6;
duration = 0.25;
retard = (textTotal-textIndex)*thisComp.frameDuration*2;
t = time - (inPoint + retard);
startVal = [100,100,100];endVal = [0,0,0];
if (t < duration){
linear(t,0,duration,startVal,endVal);
}else{
amp = (endVal - startVal)/duration;
w = freq*Math.PI*2;
endVal + amp*(Math.sin((t-duration)*w)/Math.exp(decay*(t-duration))/w);}