Upper front teeth are typically 10.0 mm to 11.5 mm long from the edge to where the white enamel meets the yellower root surface. (That is usually the gum line unless there has been gum recession on the tooth and the tooth looks longer than it is due to the recession of gum and bone from the root.)
If your big front tooth is shorter than 9 mm, you are wearing past where a tooth would be reduced to create a veneer or porcelain crown for that tooth. In other words, a tooth only 8 mm long has be broken and worn past what any dentist would do to make it properly restored. Any shorter than 8 mm means the end result is compromised when the tooth is restored.
The lower front teeth are usually around 7.0 mm to 9 mm long. Again, this is tip of tooth to where the root starts. If the tooth is worn shorter than that then again, the tooth is severely damaged. Wear on lower front teeth is more severe as these are tiny teeth to begin with and a loss of tooth structure is very compromising to a proper restoration of the teeth.