I received my kettlebell last week (16kg) and started with two arm swings for 10 reps then to one arm swings for 10 reps each. Already I was breathing hard. When I did snatches (10 reps each arm) I was pretty whipped.
I consider myself to be in good condition. I work out, plus I teach a boot camp class 3 times a week. This was the lightest kettlebell and I never did more than 10 reps!
Be aware of this when ordering kettlebells. Remember you can always increase the weight later.