user_id
event
time

SELECT COUNT(t.user_id) cnt_session, t.user_id
FROM time_user t
WHERE ROUND(
    (
        (
            SELECT t1.`time`
            FROM time_user t1
            WHERE t1.user_id = t.user_id AND t1.`time` > t.`time` LIMIT 1
        )
         - t.`time`
    ) / 60
) > 5
GROUP BY t.user_id
ORDER BY t.`time` ASC

CREATE TABLE `time_user` (
  `user_id` INT(11) NULL DEFAULT NULL,
  `time` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP,
  `event` ENUM('Y','N') NULL DEFAULT NULL,
  INDEX `user_id` (`user_id`),
  INDEX `time` (`time`)
  )
  COLLATE='utf8_general_ci'
  ENGINE=InnoDB
  ;

select avg(length)
  from (
        select user_id, snum, TIMESTAMPDIFF(SECOND,min(time),max(time)) length
          from(
               select user_id,time,
                      @snum:=if(@user=user_id and TIMESTAMPDIFF(SECOND,@ptime,time) <= 60*5,
                                @snum,@snum+1) snum,
                      @user:=user_id, @ptime:=time
                 from time_user,
                      (select @ptime:=0, @user:=0, @snum:=0) A
                order by user_id, time
              ) B
         group by snum
      ) C

select t.user_id, count(t.user_id)+1 as cnt_session, (SELECT count(DISTINCT t2.user_id) FROM time_user t2) as cnt_users
from time_user t
WHERE ROUND(((SELECT t1.`time` FROM time_user t1 WHERE t1.user_id = t.user_id AND t1.`time` > t.`time` ORDER BY t1.time ASC LIMIT 1) - t.`time`) / 60) > 5
GROUP BY t.user_id
ORDER BY t.`time` ASC