As you did not put into detail on how "simple" it is and what it looks like, I put 5 days allowance at most, and may take a week if it's longer than I think it is (60 seconds) and have more than 2 characters. Are background and character references pre-provided? Is it action-y or just full of movements in general?
Also, I would like to ask what its purpose is for, since the style and look of the animation is dependent on that. I mostly work on Toon Boom, sometimes Flash, but rarely in AfterFX since it's more for post-production and combining assets.