Incisal liability on an average in the maxillary arch is:

The exchange of incisors: During the first transitional period, the deciduous incisors are replaced by the permanent incisors. The mandibular central incisors are usually the first to erupt. The permanent incisors are considerably larger than the deciduous teeth they replace. This difference between the amount of space needed for the accommodation of the incisors and the amount of space available for this is called incisal liability. The incisal liability is roughly about 7 mm in the maxillary arch and about 5 mm in the mandibular arch. Ref: Textbook of orthodontics 5th ed Bhalaji pg no 59