In my opinion it depends on the fact if you are using an API or not.
I had an API and exaggerated it a little bit (i randomly followed about 1200 within 3 days and obviously got banned...).
Now i just open a list (for example one of those "twitter-folks that follow back") and follow 80 per hour, but no more than 400 a day without API, just by myselve.
Maybe twitter considers also the amout of people that follow you back and the amount of those that you are unfollowing after you waited some time to check wheater they follow you back!