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 belowChange 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