I'll space closer together at the bottom and increase as you go up.  Couldn't say an actual length but on a T post usually start at 5 nubs, do that twice and then do 6 and then 7.  One thing is once they start doing that crap they usually don't stop no matter how good the fence is.  I've seen them weasel through a perfectly tight and spaced fence.  Adding some stays between the posts may help.