Remember me is deep in the bowels of FormsAuthentication - google 'createPersistentCookie' to see various explanations.
To set some other length of time (I think it's set to 6 months) you'd have to write your own login control and so the ticket and cookie yourself.
Another way would be to get rid of the 'remember me' checkbox, and instead change the default forms cookie expiration length - see here http://msdn2.microsoft.com/en-us/library/1d3t3c61(VS.80).aspx
The timeout value you could set to something other than 30 mins., you also have the sliding option.