You can create a variable in a variable input box, a button with a set variable action or a control field in a list box/multi box that will control your expression(s) to show either 24 or 72 months.
I suggest that you post your application so that someone can show you how to achieve one of the above for your model.
Regards
Jonathan
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein