Theres no "correct" answer to an extent for this, it all depends on what your trying to achieve, if its movement I suggest just moving and focusing on footwork, punching technique and timing for as long as it takes you to get into your stride. If your tryng to work on stamina I would do two minute rounds, make each one differnt, look for combinations and constant punching, focus on speed and power on some then a combination of the two. Increase the amount of rounds you do gradualy, start with about 10. Theres no point in doing this, as chris n. said, until you have your technique right, otherwise all the work you do will be a waste of time in the scheme of things.