I would look at a firm that will give you the best chance at making it to “Indy status”. Look for a firm with lower hurdles if you are 110% sure you are going Indy. I feel that Ed Jones will train you well and your hurdles are relatively low. MSSB is a way better firm and will train you a little better with affluent clients and the salary is a lot higher. The hurdles are higher but tier 3 is laughable. I would go Ed Jones if you are certain you will leave and MSSB if there is any chance of you liking the firm and bringing in enough assets to survive.
As far as how long. That depends on you and whereyou will go after being at a wire. Some will say 10 million if you go to an already established Indy office or more like 25 million if you are going to hang your own shingle.

Just my 2 cents…