The way PokeRus works is a little confusing at first, but it is VERY beneficial if you EV train.
Basically, PokeRus is a virus that your Pokemon can catch from battling wild Pokemon. I think the probability of catching it is like, 3/65000. What PokeRus essentially does is double the EV gain at the end of every fight to the Pokemon that is infected with it. For example, if you fight a Seaking, they give 2 EV's in the "Attack" stat. If the Pokemon you used is infected with PokeRus, it will double the EV gain and after KO'ing a Seaking, it will gain 4 EV's instead of 2. This is very helpful if you are EV training Pokemon for competitive play, since it can greatly reduce the amount of time you'd have to put into training one stat.
For how long PokeRus is contaigious, no one really knows the exact time frame, but I've always assumed it's contaigious for ~24 hours, but others have claimed it can last up to three days. At the end of that time frame, the virus is no longer contaigious and the Pokemon that was infected with it will have a smiley face when you look at them in your party. My tip on how to keep PokeRus alive in your game, is to infect a box full of useless Pokemon (I used abras, it doesn't matter what you use) to preserve it. As long as a Pokemon is locked away in a box, they will not lose the contaigious effects of the virus, however, if they are left outside of the box (ex: in your party), they will lose the contaigiousness of the virus like normal. I have a box full of Abras with PokeRus, so when I begin to train a new team for competitive play, I have the ability to infect everyone with the virus to make EV training go faster.