2-4lb a month seems reasonable if you have a decent amount of body fat to spare I’d say. Again, from experience I think that you need to lose weight slower as body fat % reduces. Not sure what the science is there either though.
I was doing a kilo a week some weeks and was going for around 3 months! Really extreme stuff now that i look back over TP and MFP. Was over 82kg and went to 71.3kg. Bodyfat at the end was hovering in the high 10% to low 11%. Must have been over 15 to 16% when starting though didn’t track that at the very start unfortunately.
If you look at what @rjessop was saying, that was something I experienced, but to an extreme. In the end I couldn’t get my heart rate up and felt like I was 90 years old and had to move around slowly. Even doing 150 watts felt like hell.
For a while I was worried I’d permanently destroyed my body, but things are back to normal and power returned funnily enough without much work. Just stopped training and started eating normally again. Interesting learning experience, I was doing the whole clean eating thing and using energy drinks to fuel workouts as that’s the only way you can really get the energy to even attempt a workout when your glycogen reserves are permanently shot!
Didn’t really want to get into my personal situation, but was hoping for more on the theoretical side of things. I do have tons of tracked data and info about my experience going hardcore earlier in the year, but would take hours of writing to even touch on what it was really like and what the numbers and power loss was and the workouts I was doing, and when the failing started etc…