I would start and drive it ~15-20 min once a week.

car storage is more than just the battery— there’s tires, gas, lube, etc. you might want to check on the internet but i’d at least drive it a little every month. Fwiw i’ve found not driving a car is as hard on it as driving it a lot.

Would it be easier to disconnect the battery? Why did you buy a car you don’t plan to drive?

I recently bought a new car but have no plans to get rid of my old one. It's at least 10 years old and still works fine.

My question is how often should I start it so the battery doesn't die.