No visa is necessary for most western countries (but please check with the Peruvian Embassy in your home country). The tourist visa you will get upon arrival at Lima Airport, will allow you 90 days in Peru. You are allowed a maximum of three 30-day extensions for up 180 days (6 months) in Peru. Each extension will cost you about $ 25.00. You might want to consider just crossing the (Bolivian or Chilean) border, if you need more time in Peru. It'll take you about two days and you'll be guaranteed another 90 days.