Hi Sam,
You can configure a command "exec-timeout 0 0" under line vty 0 4 and under line console 0 to retain the session forever but I will not recommed you to keep it up forever and occupy the vty lines but instead you can configure to increase the time out with same command exec-timeout 30 0 which will keep the session to be active for 30 minutes or more also if you want.
The first value is for minutes and 2nd value is for seconds in this command.
To specify no timeout, enter the exec-timeout 0 0 command.
You can also check this link for more details
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tcf_r/cf_03ht.htm#wp1035605
HTH, if yes please rate the post.
Ankur
