The most I lost was about 20 lbs. in one month, or about 4-5 lbs a week. The most adverse affect that I received (that I know of) was a lot of stretch marks, but it's possible that I lost muscle too and had to rebuild it.

I know that the question wasn't asking me what my most weight loss was, I am just trying to use personal experience to say that I didn't notice anything unhealthy about that level of weight loss. And if it is not a healthy weight loss, why?

20% in a year seems like a ton, but if they're 60% bodyfat right now, it'd be more reasonable than if they were, say 25% bodyfat right now.

for continual long termfat loss, and not losing any muscle mass in the process, I doubt you can do much better, on average, than 1lb of pure fat loss per week. at the beginning it'll go fast, but at the end it'll be a crawl, and losing lean mass will be a little harder to fight off

*edited to clarify*

Hmm that is a tough one. I think it depends on the person's state of health. There are times when people are encouraged to lose extrordinary amounts of weight rapidly as is seen with gastric bypass patients, but this type of loss comes with a price as the person is truly malnourished and without proper medical care could become seriously ill. morbidly obese people over 30 losing weight at a rate like that should always consult a physician and a possible a dietician/ nutritionist, and a physical trainer, especially before starting a new workout. Many obese people over 30 have underlying health problems they are unaware of. I think on average 1-2 lbs a week is a reasonable rate. Very overweight people may lose more at first but will level off in a diet regiman designed to lose at that rate. Hope that helps.