I would make the change when one of the following two things happen:
1. Jimmy and Petr become a problem in the locker room. According to some quotes currently both guys see this as a challenge and are supporting each other.
2. One of them shats the bed. Other than that keep the rotation going, I think the decline of Howard towards the end of seasons has been due to logging too many minutes.
I think J-Ho's body just cannot handle 60+ starts a year. If both of these guys can play at current level (they are almost mirror images stat wise), missing a couple of games will help them rest and hopefully keep on the same pace.