Android ArgbEvaluator calculate fraction

I've a little difficult in using ArgbEvaluator object. I want to change the color of view from one color to another color in specific duration.So I write this code,it works well.

ValueAnimator animator = ValueAnimator.ofArgb(Color.RED,Color.BLUE);
    animator.setDuration(5000);
    animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
        @Override
        public void onAnimationUpdate(ValueAnimator animation) {
            int color = (int) animation.getAnimatedValue();
            view.setBackgroundColor(color);
        }
    });
    animator.start();

But I want to use ArgbEvaluator to animate like that.

public void setColor(fraction){
    int color = evaluator.evaluate(fraction,startColor,endColor);
    view.setBackgroundColor(color);
}

The above setColor method will be called agian and again for specific duration(eg-5000ms). Here is my problem. I don't know how to calculate fraction to change color within specific duration. I want to know the calculation formula of fraction.Thanks.