Rule must be activated (on agenda) to be eligible for invocation by fireAllRules, so multiple invocation of fireAllRules without modification to the session will not have any consequence. Activation happens on insert/update/retract operations on session. So, in your case you should insert or update a fact on session whenever human task is created (for instance with onEntry script) and fire activated rules. To trigger fireAllRules automatically you could use AgendaEventListener registered on the session, example you can find here
HTH