Exercise old 4 ton HVAC I am now using Mini Splits to cool with.
 

I have a 4 ton AC only system and natural gas heat. Normally the AC stays off all winter then it get used all summer.

Now I put in Mini splits and a ECM fan motor. I am not using my 4 ton AC at all so far. I love the power savings and I am even keeping the house cooler that I did before and still saving money.

The question is how often should I run my old AC and for how long just to keep it in working order?

There should be no problem leaving it unused.