"When is the best time to upgrade your rank from EA to Team Leader?"
I would say in answer to this, when you see that you will benefit from the rank.
If you don't have an active team then there is not a great deal to be gained by upgrading to BTL. You can't benefit from matching VP if your team doesn't generate much VP. You can check on your Scoreboard page to see how much you could be making if you upgraded. If that amount is less than what it costs to upgrade, then my advice is not to worry about upgrading yet. Of course you do benefit from receiving CSAs each month that you are a TL, but my experience is that most of these are inactive anyway.
My recommendation is to concentrate on working the business making sales and growing your team. Be a great sponsor and help your team to grow. When you do the work of a team leader, your team will grow and prosper and you will gain TL status without having to worry about the timing of it at all.
"When is the best time to upgrade your rank from EA to Team Leader?"
I would say in answer to this, when you see that you will benefit from the rank.
If you don't have an active team then there is not a great deal to be gained by upgrading to BTL. You can't benefit from matching VP if your team doesn't generate much VP. You can check on your Scoreboard page to see how much you could be making if you upgraded. If that amount is less than what it costs to upgrade, then ...more