var slideWidth = 900;

 $(function() {

   $("#showNextSlide").click(function() {
     var position = $("#slideContainer").position();

     var newX = position.left-(slideWidth);
     var tweenTime = 1000;

     if (newX < 0-(slideWidth*5)){
         newX = 0;
         tweenTime = 3000;
     }

     $("#slideContainer").animate({left:newX}, tweenTime );
   });
 });