I can only speak to my own experience, Nature Nut. Just this week I had an incredibly easy one where I took over 100 minutes, Just as it finished loading, the power went out. Sometimes I get called away from thte computer for one reason or another or to run an errand. Then of course there is the infamous brain stall where I can't see a pattern to save my life. I'm certain others may have different reasons.
"Great spirits have always encountered violent opposition from mediocre minds." ~ Albert Einstein