The simplist way is from cron, or similar

  update table set turns = turns+ x;
  update table set turns = max where turns > max;
  commit;
[download]

As a next option, if you don't have people logged on for days at a time, you can just give them their turns on login -- keep track of their last login date, and when they log in, give them (turns_per_day * (datediff(systime(), last_login))).

  update table set
    last_login = sysdate(),
    turns_left = ( $turns_per_day * datediff( sysdate(), last_login ))
  where user_id = ?
[download]

update:: bah ... the second example didn't take into consideration order of operations:

  update table set
    turns_left = ( $turns_per_day * datediff( sysdate(), last_login ))
+,
    last_login = sysdate()
  where user_id = ?
[download]