It will depend on a number of factors, how often the car is used, where it is parked, weather conditions and how the car is washed. A machine wash will outstrip polish faster than a hand wash. As a rule, polish metallic paint every 8 weeks although Autoglym Ultra Deep Shine is a polish used on dark paint to help eliminate swirling and give more depth of shine. Therefore I would recommend using Autoglym Ultra Deep Shine and then applying a coat of Autoglym Extra Gloss Protector. For further advice why not visit the Autoglym site and use their "ask an expert" function: http://www.autoglym.com/enGB/AskExp...

There isn't a specific polish for metallic paint within the Autoglym range.
There are a number of polishes that are suitable.
It more depends upon the colour of your car. For dark colours Ultra deep shine is ideal, for lighter colours it is recommended to use super resin polish followed by extra gloss protection for best results.