It will be very, very busy. I would try to go on Friday right when the park opens. We stood in line for Darth Vader for over an hour last year. The line moves slowly, and Vader has to take frequent breaks because you can imagine he gets VERY WARM in that mask.
In fact, if Vader is handled the same way other Disney full-mask characters are, they only work 15 minutes of each hour (though there may be multiple CMs in identical costumes rotating in and out).