Fastest I've lost a bike to theft was about 3 minutes. Parked it in the back yard of a house I was working in. Seven foot solid fence around the yard. A couple of minutes after I was inside I got nervous and went back out to lock the bike. Gone from a yard with an unlocked gate. The Irvine, CA police said I was 3 blocks from the High School and theft of unlocked bikes was endemic. The officer was sure someone had seen me going into the yard before and just kept checking until I felt comfortable and left the bike unlocked.

Now, even on group rides, I carry a U lock and lock up when we stop for anything.