smrobs and loosie r right....6 weeks is too long to wait for some horses....like our barefoot Belgian. She started cracking and chipping off huge hunks a few months after we got a new farrier closer to where we moved to... this was new and totally unexpected....so we called the old farrier back. The new one was trimming her long, but she grows alot of hoof and must be kept short. Even with her old farrier now doing her feet again, she still cracks/chips at about 5 weeks after her last trim.
6 week trimmings are not universal. Some need longer, some need shorter.