I don't know what qualifies a climber as no longer a "rookie," but I do know that meeting the requirements to join the ISA is not a reflection of climbing experience or skills. The ISA is an organization for anyone in the field of arboriculture. There are lots of jobs in this field that don't involve climbing, and a great many of the people who do them are members of ISA.
To answer the question about how you know if you're no longer a rookie:
If you are looking at a tree job that looks too dangerous for you, pay attention to your gut and don't do it. I would say that what you see on the job and decide to do or not do is more important than knowing whether or not you're a rookie. Know your limitations -- everyone has them. The question of safety is whether you pay attention to them.
Might I suggest that you post that question on an arborist Forums like TreeBuzz, which is specifically for arborists. The TCI Forums, while helpful to a great many people who do tree work, is geared more toward recreational climbers.
Thanks for asking!
Patty
