General:
* Constant: Emphasizes the unchanging nature of something.
* Unending: Highlights the lack of a foreseeable end.
* Continuous: Stresses the lack of interruption.
* Persistent: Focuses on the determination or stubbornness of something.
* Relentless: Implies a forceful and unstoppable quality.
More specific:
* Unceasing: Emphasizes the lack of pause or break.
* Nonstop: Highlights the lack of any time off.
* Never-ending: Similar to "unending," but with a more dramatic feel.
* Chronic: Used for conditions or behaviors that are ongoing and persistent.
* Unflagging: Implies unwavering determination or effort.
* Insatiable: Often used for desires or appetites that are never satisfied.
Figurative:
* Pestering: Suggests a constant annoyance or irritation.
* Indefatigable: Implies an endless capacity for work or effort.
* Perpetual: Means "lasting forever" or "occurring repeatedly."
The best word to use will depend on the specific context and the feeling you want to convey.