How to get from terminal b to terminal a and how long?

Sky line is the best

Skylink

Skylink is fast, free. Much faster than leaving terminal and enduring security re-entry process. Times vary, but 10-20 minutes should be expected.

Take a bus outside (have to go back through security) or take the Skylink Train from within the terminal. Timing will vary. But I'd say you should be able to between terminals in less than 20 mins.