I've done 5:2 a few times over the past decade, usually with some success, though it takes months before weight comes down very much. I think the most I managed to lose was around 11kg in a 12 month period, or around 2/15th of my starting weight at the time. As I've got older, I found more issues with plateauing, and starting from a higher weight each time probably didn't help. I've been firmly off the wagon for several months now, but really need to start again soon.