Answer by KenJ ·
As always with a licensing question, contact Microsoft directly for specifics. They have contact information on their pricing and licensing page - https://www.microsoft.com/en-us/sql-server/sql-server-2017-pricing
Microsoft SQL licenses are typically perpetual - you pay for it once and your license for that particular combination of SQL Server version, edition and core count does not expire. Licensing does not come with support - that is negotiated separately as an enterprise support agreement or purchased on a pay-per-incident basis.
Microsoft also have some licensing add-ons that do expire, such as Software Assurance, that provide additional benefits around flexibility of license use and software installation (such as time-boxed upgrade rights, unlimited virtualization, license transfer rights, high availability rights, etc).
One non-perpetual SQL Server license is the Service Provider License Agreement (SPLA) for which you pay-as-you-go each month based on your version/edition of SQL Server and the number of cores you are licensing. Unless you are providing SQL Server access in a SaaS environment, you are unlikely to encounter this type of licensing, but you can find out more here - https://www.microsoft.com/en-my/licensing/licensing-programs/spla-program?activetab=spla-program%3aprimaryr2