You can store refrigerants for as long as you would like. The gas will not go bad as long as there is not a leak in your cylinder. If your cylinder is completely sealed you can hold onto your refrigerant for years without any worry. There are many people who still have a hold of R-12 refrigerant from the early 1990s. Product is still good too!
Some people will keep an eye out for upcoming phase outs of refrigerants and if they see a potential big phase out coming they will buy a few cylinders of that refrigerant and sit on it for years and years. They’ll either keep it for personal use or hold on to it until the price sky rockets and then sell it for a huge profit.