Harry Grant plays 80 nearly every week. He hasn't played fewer than 71:
The NRL profile of Harry Grant including career statistics, season stats, games, tries, goals, tackles and fantasy scores.
www.nrl.com
Cook and Robson also play 80 minutes nearly every week:
The NRL profile of Damien Cook including career statistics, season stats, games, tries, goals, tackles and fantasy scores.
www.nrl.com
The NRL profile of Reece Robson including career statistics, season stats, games, tries, goals, tackles and fantasy scores.
www.nrl.com
It seems to come down to the quality of the second best hooker in the squad rather than some arbitrary rule about the number of minutes some merkin is capable of. They are full time professionals ffs. There's 10080 minutes in a week, so they can play a full game and still have 10000 minutes to rest up for next week. Yes they will be more fatigued at the end of the game than at the start, but that's true for every player. The question is whether they are too fatigued to remain on the field, or whether someone fresh off the bench would be doing a better job. In the case of some nines, their coach seems to think they are better off playing 80. This includes Bellamy and Payten.