At my bike co-op we ended up with a u-lock (giant brand, about 35 dollars new) that we didn't have a key for. So a friend videoed me using a cheap long arm bolt cutters from Harbor Freight to see if we could cut it.

Put one arm of the bolt cutters on the ground, put the lock between the cutters, and pushed down. After about 20 seconds, it snapped. Mind you I was being careful because I didn't want to fall and hurt myself. Put the video on my FB feed and a bunch of my bike friends said I cheated by putting it on the ground.

I then took another video of me holding the bolt cutters with one arm against my chest and snapping the lock that way without using the floor. That time it took me all of 5 seconds to snap it. I didn't hesitate and just pulled it as hard as I could quickly.