In my experience, everyone should be buying RIs or a Savings Plan, right as soon as they start using instances. At the very least it's an incentive to plan to move to cheaper tech after 3 years. But it also takes at least 3 years for businesses to get serious about trying to save on their spend. And you can change instances or types during that time. Most people don't end up changing anything, though, and wish they had been saving.