I have worked my way up to 15lbs set squarely in the small of my back (it is a core exercise after all) for 3:15 with good form. That is to failure mind you, I'm shot by that point.

I had gone to 4+ minutes with body weight, then the "brain-burn" torture for that long got me to add resistance. I tried a 20lb weight vest, but it seemed to put more emphasis on the shoulders than it did the core since it rode higher. Then I started having my wife place 15lbs in the small of my back as soon as I got into plank position.

Is that at least respectable? It sucks balls and it's funny how your HR shoots up in only 3+ minutes of exercise!