General:
* As focused as a laser beam.
* As unwavering as a lighthouse in a storm.
* As persistent as a dog with a bone.
* As committed as a moth to a flame.
* As loyal as a faithful friend.
* As steadfast as a rock in the ocean.
Specific to different areas:
* As dedicated to their art as a sculptor to their marble.
* As dedicated to their students as a gardener to their flowers.
* As dedicated to their cause as a warrior to their king.
The best simile for "dedicated" will depend on the specific context and the kind of dedication you want to emphasize.