capacitator needs?

[boardsportsrule]

ok, im making a box and i want it to fade off when i turn it off....i will be getting these LEDS (http://123leds.com/product_info.php?products_id=52) and i will be running off a 5V or 12V power source. i would like it to slowly fade, for about 5 seconds. please help me either by calculating how much, or showing me how to find what size i need...thanks! (ps. i dun wanna use more then one cap per led, and if i could have them all off one cap that would be SICK, cuz you will be able to see inside this box, and i want it to be as tidy looking as possible!