About 5 weeks for 50 pounds, 4 for 40 and so on. At least. That's what I heard! I just got it today!;)
I have an ab circle pro and I've been using it for about a month and I and my partner have noticed my waist size is smaller by 4cm and I am a lot firmer.
I would say the speed it works depends on how you use it, how often, for how long and with what other exercises and diet.
But I wouldn't say it's much use just using the ab circle pro on it's own since it doesn't raise the heart and body temperature fro long enough to start using fat stores if your only using it for 3 - 5min a day.
I use mine for 5min a day along with doing 15min of pilates and it definitely helps.
But if you want to measure results I found that I got heavier on the scales because of the muscle I gained but thinner when I actually measures my body using a measuring tape.
So I wouldn't trust the scales.
But yes it is a great device for slimming your waist area if you use it properly and regularly with other exercise that allow you to burn the fat stores you have.
The recommended daily exercise amount is 30min on top of normal activities and a proper diet.
I only do 20min a day and it seems to be enough for me.