Aggravating Patrol Mission

[vek73]

I've read other people having this issue, and tried to follow the advice they were given, but I still can't seem to get around this problem. On my current patrol (my first career ever), I have a patrol mission. "Patrol off the island of Honshu." I headed directly to the mission icon, and I drew a circle around it with a 100nm radius (I read to stay within 100nm of the spot). Once there, I basically laid out patrol patterns within the circle, and made sure not to leave the circle. I've been in the perimeter for around 72 hours, I've sunk several ships, sent contact reports, even tried sending status reports, and the damn mission won't complete. Is there something I'm possibly missing? How long do these patrols last?