The maximum bound for most gTLD registries is ten years (have a dig around the ICANN website if you'd like to check), not fifteen. Sure, there might be a ccTLD that allows you to register a domain for longer than that, but I'm not aware of any, and ccTLDs usually have lower maximum renewal/registration bounds than the gTLDs.

If a registrar is letting you renew a gTLD for 15 years, they're basically sitting on a third of your money until the domain's expiration date comes up.

You don't have to do it all in one shot but you should make sure it gets paid so you don't lose access.