on ............removeclass addclass

on ............removeclass addclass

I want to change class slow speed, I want values i.e. 1000 or something else not slow or fast

where can i write this code

<script type="text/javascript" src="js/jquery-1.8.3.js"></script>
<style type="text/css">

div.d1_1
{
background-color: #9F0;
height: 100px;
width: 100px;
border: 1px thin #000;
}

div.d1_2{
background-color: #639;
height: 200px;
width: 200px;
border: 1px thin #000;
}
</style>

<script type="text/javascript">
$(document).ready(function() {
    $('.d1_1').mouseenter(
    function()
    {
        $(this).removeClass("d1_1");
        $(this).addClass("d1_2");
    })
   
    .mouseleave(
    function()
    {
        $(this).removeClass("d1_2");
        $(this).addClass("d1_1");
    })
});
</script>