The post restoration does not only depend upon length but also on type of tooth, condition of remaining teeth, length of crown required and other biological factors etc., which can not be enumerated on e mail. Please leave this decision to the Prosthodontist.
In general, it is recommended that any root which can be restored and a full tooth can be made using post and core, it is better to go for it. Not having a tooth in position can also harm your adjoining and opposing teeth. The life of a capped post and core tooth may range from few months to many years depending upon usage and expertise of the operator.