how long does a pokerus last?

Its only contagious for a little while, but I don't now for how long.
I think it depends on how many steps you take.

well......forever i think......i dont heal my pokes at the center and i dont save with them in my team.....i leave them in the pc

1) Please use the Q and A thread, this question has been answered several times, and is even referenced on the first page of that thread in the "rules" post.

2)
[source: Bulbapedia]
A Pokémon with the Pokérus will infect other Pokémon in the Trainer's active party while walking, battling, or healing at a Pokémon Center.

Pokérus will not infect other Pokémon if it's stored in the PC, and will only become immune while in a Trainer's party.

Additionally, in Diamond and Pearl Pokérus can infect eggs that have not hatched. When the Pokémon is born, it will be infected with the Pokérus, if it has not worn off before it has hatched.
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Pokerus will stay with a poke forever, however what you should be asking is how long does it stay contagious or "transmittable" to your other pokes. Whenever your ingame clock resets to midnight, any poke in your party will become healed. The double EV effect, is in play, for the rest of their natural, and unatrual little digital lives, however, any poke even in egg format will become non-contagious at midnight.

So keep a seeder poke with the virus in the box at all times, at minimum. Then just try to keep a few pokes with the virus that are fully trained in the box as well.

don't know exactly but i do know that it does leave because one of my friends has a lot of them and they left.

Pokerus...
Once a pokemon gets infected with it, the effects are everlasting. However, a pokemon can only give it to another pokemon within 3 days of contracting it unless you put it in the PC, in which case there is no limit. But once you take it out, the 3 day count will continue. Also, the pokemon with pokerus you put in the PC will not give it to the other pokemon in the PC... however getting pokerus is very unlikly. However not as unlikely as running into a Shiny...

ive gotten pokerus in almost every game ive had....

I believe it depends how the steps you take
However, if you keep an infected Pokemon in the PC, it will stay with it forever.
I have several infected pokemon that i keep in a box if i want to pass it on to another pokemon in training.
Pretty cool =)

I wish my Pokemon could get Pokerus.

go to the trade thread and look for a trader that can give you a virus-infected poke.

Would but first I gotta get Wi-Fi... The McDonalds at where I work doesn't even have wifi yet....

Leave it in one of your Pokemon boxes. It spreads in there. Then keep switching it up as more and more get infected, more and more can spread it. Keep it going as long as you can.