You need to add a key at the start of the animation for that item (use part/ group selection mode) right click and add keyframe.
Then you can duplicate that key and move it to the point you wish it to stay visible until.
Finally add another keyframe either manually or using auto keyframing to hide the item, as far as i know there is no easy way to fade out. You could use an appearance that has a transparency/ color density and adjust this value during the animation i suppose but for some appearances it just isn't an option as far as i know so you cannot fade it unless i am missing something.
Alan is correct; there is currently no way to animate the fading of parts in Visualize...yet Sit tight and we have something cool coming quite soon. The workaround he provided can work, depending on the appearance used on the parts you want to fade.
Saurabh, this is a great feature to request and please keep them coming!
I think I can make this work for the time being. I will have to play with the camera angles to ensure that the part being hidden is not in the view when it suddenly disappears though.
Thanks for the input Alan!