Your commercial license for the Structure plugin (including Starter licenses) typically has no expiration date, so it's good to use forever. However, it has Maintenance Expiration Date which limits which versions of the plugin can be used with that license – you can only use the versions released prior to that date.
To use versions released later, you need to purchase maintenance renewal, which extends your maintenance expiration date one year forward – independently of the date of purchase.
Date license purchased
License expiration date
Maintenance expiration date
Products and terms allowed by the license
All versions released prior to 2013-01-01 can be used indefinitely
Maintenance renewal purchased
2012-12-10 (doesn't matter)
Renewed license maintenance expiration date
All versions released prior to 2014-01-01 can be used indefinitely
Evaluation and temporary licenses have an expiration date, after which they just stop working – they allow to use the product before the specified date.
Make sure you renew evaluation or get another license key before expiration.
If the currently used license becomes invalid (for example, because it is expired, or because you've upgraded to a version of Structure that's not covered by the current license's maintenance), then Structure plugin will function in read-only mode.
The users will be able to view structures, but they won't be able to make any changes until a valid license is installed.