Method Question - Mailing list pgsql-novice

I'm making a layout for a timecard, if a user could clock in/out just once
per day, it wouldn't be a problem, but I need to assume no limits, I
currently have it setup


CREATE TABLE timecard ('id' SERIAL,'employee' varchar(100),'time'
timestamp, inorout varchar(5));

 id | emp  |        time         | outorin
----+------+---------------------+---------
  4 | 9826 | 2004-01-27 06:08:52 | i
  5 | 9826 | 2004-01-27 06:19:54 | o
  6 | 9826 | 2004-01-27 06:20:05 | i
  7 | 9826 | 2004-01-27 08:15:13 | o


Using that as example of the data, how could I tell how many hours have
been in between each i and o? This question stumpeth me. Please help.