Your bird's beak grows continuously, like a fingernail or hoof, but a healthy bird should never need to have his beak trimmed. If your bird's beak-pruning habits change, and the beak grows long and brittle, it could be a sign of illness. A bird needs to have his beak trimmed when the upper and lower portion don't line up properly, interfering with eating. A misaligned beak is also more prone to cracking or breaking, a painful and dangerous event because a severe hemorrhage could occur. So, a cracked beak should be treated quickly.
For more information, please read the story Grooming Your Bird.