A Couple of Quotes from the Treebuzers site:
1st Quote by Sizzler:
"Here's a question, i set a line in an oak at my old man's house to practice. the TIP was at 45 ft so the limbs were a little on the small side. my TIP was plenty strong, but after climbing it about 10-15 times over the course of a few days i started to wonder if each time i was further weakening the limb so i pulled my rope. is this a justifiable concern or is the limb probly just as strong as it ever was? or, is there no way to say since you havent seen the tree? thanks!!
2nd Quote by Leon:
I'd guess that the limb is just as strong. In fact, when trees are subjected to repeated stress by wind, or their own weight, they grow extra wood to compensate for it. If you were to climb daily on the same limb for a year you might find a relatively greater growth of new wood on that limb than on similar nearby ones. Of course, if you're climbing on the same TIP that often make sure you're using a cambium saver of some sort.
end of quotes