Logged in length of time

Once a user has logged into a site I have built, I would like to give them say 30 minutes of idle time before the server logs them out... currently it seems to do so at about 10 minutes...

How do I do this?

I have tried to search the web for this ability, but can't think what it's called and therefore am getting the worng thing returned.

I'm using php and MySQL    

Thanks.