write and force site to use custome Cookie

a friend of mine has a website and doesn't have access to it's files (sort of classified somthing like that ) , the problem is when a user log in it just takes him 5 minutes and logged of because Cookies expiration is 5 minutes , he asked me to make a land page where the visitors can click on a button that leads them to the site , but also put a custom Cookie file in the browser with expiration time 5 days or a week or a long time the 5 minutes :) , i really don't know how , can you help me please !