How to keep user logged in for long time in ASP.Net

Hello ,

I want to implement user login function , that allow user to login after couple of weaks without asking username and password again. I have read many article and got that it is maintain by Cookie but don't know how to implement this one. A good example is seen in gmail, facebook   http://www.aspforums.net  that allow user logged in  for couple of weaks. I also want to maintain a cookie table for just keepig the records of user logged in from different system. 

Please give your useful suggestion.