"I'm a buyer's agent with a team. I know that as a buyers agent I will always be dependent of a team to provide me lead. How do I know when its time to quit and learn how to become a listing agent?"
Not really a straight forward question. If the existing situation you find yourself in meets or exceeds your financial needs you may consider staying.
Flip side, you aren't really building anything.
You may find yourself without that 'job' and then what? You are back at ground zero. The economy will cycle down. When that happens the first thing that will happen to 'teams' is that the ad spending will go away. Followed shortly by the buyer agents being let go.
For all those reasons (and more) you will be better off being a listing agent who is dependent on his on wits.