I've read a lot of different things, but in my experience, you want to cap your activity at:
- 100-200 unfollows an hour
- 200-300 follows an hour
- MAX 1000 unfollows and 1000 follows a day
It is advisable to stay below these numbers - I've never met or exceeded them myself, but if you stick to 100-150 unfollows an hour and 150-200 follows w/ those maximums, you should be fine.