How long is the duration of Pokerus? Some people say that Pokerus cease to be infectious after midnight when they are in your party, others say there is an invisible timer that only advances when they are in your party, and again others say it is regulated by a stepcounter.
What is true now?
What Ausgirl said is right; it depends on the strain. When infected with the virus your pokemon is given a 'pokerus' value which goes down by 1 each time the game's clock reaches midnight if they are in the player's party. When this value reaches 0 then your pokemon is 'cured'. So this means it will last indefinitely if left in the PC. I think the potential maximum is like 15 days in the party. If you're worried about your pokemon being 'cured' just make sure you deposit them into the PC before turning off your game (or if midnight is approaching while you are playing!)