Breath weapon says "once every 10 rounds"? Eligible. Breath weapon that says "once every minute"? Not eligible. Are they the same amount of time? Absolutely! So the game really does make this sort of distinction about units of measurement for durations.To take a metabreath feat, a creature must have a breath weapon whose time between breaths is expressed in rounds. Therefore, a hell hound (which can breathe once every 2d4 rounds) can take metabreath feats, whereas a behir (breath weapon usable 1/minute) cannot.