You're probably not contagious any more; besides, it's not as easily passed on as something like a cold, as you don't generally cough and sneeze with it-you'd only likely pass it on by sharing drinks/cutlery etc.
If you feel well enough to go to work, then go, if you don't, then don't and get a note from your doctor.
It can last anything from weeks to months as well; when I had it, it was a good six months before I was really feeling much better and a good year after that before every cold I got wasn't knocking me back again. It can be a horrible thing to have, and if you don't feel up to things then the best thing to do is get plenty of rest and take it easy.