It is common to polish for extended periods of time (hours) on the vibratory polisher. This is most common for very hard materials like engineered ceramics. The effect of over polishing on metals it that you can begin to introduce relief or edge rounding where the specimen meets the mounting compound. This can also happen if the specimen is comprised of various phases/materials of varying hardness. A good example is a PCB where you might have soft materials like tin or copper alongside harder materials like ceramic.

The best practice is to play with times relative to your specimen and analysis. Checking the specimen on an optical microscope every 30 minutes is helpful when developing procedures for vibratory polishing. Also, keep in mind the colloidal silica will crystalize if allowed to dry out or dehydrate. I hope this is helpful.