General:
* Permanent
* Enduring
* Durable
* Everlasting
* Eternal
* Abiding
* Persistent
* Continuing
* Unending
* Indelible (especially for marks or memories)
More nuanced:
* Long-lasting
* Sustained
* Chronic (for conditions or problems)
* Immutable (unchanging)
* Invariable (unchanging)
* Perpetual (never-ending)
The best word to use will depend on the specific context and the desired nuance.