If you want that terminal to remember your password for ever during its life time follow these steps.
1. Type at terminal
sudo visudo
2. Edit as shown below Change
Defaults env_reset
to
Defaults env_reset,timestamp_timeout=-1
To make terminal ask for password every time you type sudo command:
Change Defaults env_reset to Defaults env_reset,timestamp_timeout=0
Source: blog.dipinkrishna.info