There is a month-to-month plan, but most folks buy outright.
While updates are free for up to six months from purchase date, there's no mandate to do so and if your version is performing well it's often wise to wait. Newer releases can always be test driven, to check for new features and quirks.😉
After the six-month period there is a charge to update. But the version you purchase will not expire. Pricing deals are often available.
https://www.nch.com.au/upgrade/index.html