Mind that this is a succesfull charge, so a succesful attack on the charge is required.
As far as i see it, a raging barbarian doesn't charge, retreat, charge again etc. Doing so costs an extra round per go:
round 1: charge
round 2: attack and move away (also provoking an attack of oppertunity)
round 3: charge
If the enemy moves too far away or too close by, the second charge cannot be made.
Also, the barbarian provokes an attack of opportunity with this routine.
I agree, if all charges are succesfull he could hold this up indefinately. But if all charges are succesfull, the bad guy(s) will be dead long before any rage would end.
To restrict the extra round to cleaves and critical hits only is too restrictive in my opinion. As comparison: the WOTC feat gives you 5 rounds extra on every rage.
Maybe i should combine the two: double your con bonus and gain an extra round for every critical hit or cleave. Some prerquisites would be in place, for instance Fury of the Wolverine.