I personally see the 3 year initiation process spent at the lodge as the most common form of entry. From what I have read about Pathfinder exploits, I am guessing that a field commission would only go out to an individual who is already exemplary in every way; in other words, not a low level character. I can see the Initiates at the lodge typically ranging from level 1-4, with the full fledged Pathfinders sitting at a good level 4-7. The truly masterful Pathfinders have levels that go beyond that, up to about 13 (this is the highest I have seen an NPC Pathfinder.)