how often??

I lost my license before for 6 mo. Just knowing I had the 'stang waiting for me when the suspension was lifted made it easier. I used time I would've been driving it doing some maintenance. Not as long as 2 yrs, but from my experience I would keep it unless you need the money for bus tickets and taxis.

EDIT: And to answer your question, start it at least weekly. My stang is no longer my daily driver and sometimes I go a couple of weeks without driving it, but I at least start it and let it idle for a bit every week if I can to keep it from crapping out.