Yes, another good point about range. Higher notes possibly more difficult to control but you’ll be able to hold them longer. @jbtsax has summarised the exercise nicely.I’ve never actually timed it but I’ve been a singer and athlete my entire life so can hold a note for a long time. I’m in agreement that a lot depends on the velocity of sound. A full out fortissimo (fff) takes a lot more breath than a piano (ppp). Also range needs to be taken into consideration too. The note your holding has a huge impact also.