Basically, it'll be added to /p [playername] and it will show how many minutes/ hours/days/weeks/months the player has been playing actively on the server. Its as simple as that.
This has already been suggested http://empireminecraft.com/threads/suggestion-detailed-p.25791/. There might be a few problems with this, and it probably won't be added soon.
I feel like that would be a bad idea because people wouldn't know if they AFK'd or just obsessively played O_O
It wouldn't be a problem to implement: On each user's disconnect from server Code: time_played = ADDTIME(time_played, TIMEDIFF(NOW(), login_time)) that's very simple and fast, done in fraction of millisecond. Calculating time_played for each player from the logs from past x years would reasonably need a separate computer and would probably last few hours - but it would only need to be done once. Performance is not a problem. Now, if kids don't tell their parents to check /p, 99.9% of them will never come to the idea themselves so, neither we would have problems with privacy issues...