What processes are involved in searching one person's luggage for three hours? You'd think that there would be nothing left to do after 10 minutes.
Even if there were nothing to do after 10 minutes, sometimes the process is dragged out so as to hassle the individual and observe the response(s) to the situation over a longer period of time.
Baggage searches physically can take substantially longer than 10 minutes if they want to examine the pieces/items individually and then question the person about each of the items one by one. When electronics are involved, it can take even longer if they want to take things apart while trying not to break the stuff. Sometimes they rummage through papers and play "curious George" about the items and then have someone else ask questions about the same, either in the same way or another way.