Mine sat in a tow yard for 6 years. All I had to do was get fresh gas to it and it started and ran perfectly. That was 4+ years ago and I've had to do some routine maintenance on it since then: points (it's 34 years old), plugs, choke adjustment and oil changes.
So based on my sample size of one, it seems if you run them once every six years they'd be okay. I exercise it when I remember - probably once every 3 months or so. If it's winter I load it up with an electric heater. In the summer I run both A/Cs.
