Not heavy enough! After 12 weeks lifting 5 x 5 and heavy, I dropped the weight to about 70% of what I had been doing and did 4 sets x 10! It was a different workout but it hurt twice as much and the pain lasted twice as long.

And that was just 4 sets. Lowering the weight even more and extending the sets would definitely give results.

Don't know personally whether sets of 50 would work long term. Maybe as a shock tactic, but having never done that sort of workout I really don't know.

I do think that when most people think high rep sets they are thinking of low weights and training for endurance. So maybe those of us who haven't done them would be shocked. In more ways than one.