It's always good to start your engines periodically no matter the size. I don't know any secret formulas but here is what I do.
If it's small motor, like a snow blower or lawn mower, I start them at least once every 3 months, if they are going to sit for extended periods of time. I also use a siphon to take all the fuel out of them while sitting.
For cars I try to start any car that I don't drive often, every couple of weeks. I will normally drive it for about 20-30 minutes, 1-2 times per month. While not driving it I keep a battery maintainer on it.
If you are going to leave fuel in your cars/small motors while they sit for extended periods of time you should add a fuel stabilizer to the mix.
Side note, I have an old Chevy that I rarely drive, but keep because I love the car. I was really good the first couple of years of starting it every couple of weeks or so and taking it for a short drive, but this past year I got lazy. The car is in terrible shape right now, other than a completely drained battery 3 out of the 4 wheels of seized and I need to replace a good portion of my brake lines. I let it sit for 7 months without starting it or driving it; lesson learned.