Depends some on your run design. Are you building a wooden frame where you have lumber running horizontally, top, bottom, or both. The most cost efficient lengths of most lumber is 8', so setting posts at 8 feet make sense.
If you are stretching wire mesh between wooden posts you can go a little further, say 10' to 12' if you have livestock like cattle, horses, or sheep pushing on it. If it is just chickens and wind load you can go a lot further, 15 to maybe as much as 20 feet. The further you go the more the fencing can sag so you will need to stretch it pretty tightly. If you are using metal T-posts I'd probably not go further than 8' spacing.
