Amongst your switches at Admin Panel -> Settings -> Switches is a "total time online" option. When enabled, each user profile will display the cumulative time the member has spent on the site. This isn't the time since registration, but rather the actual time viewing the site -- so a regular visitor who's been registered for a year may have 3 days of total time online.

The member list will also allow sorting by total time online, so you can see who's been around the most. This can be useful in picking moderators or rewarding faithful users.