I'm a fan of "per X" where X is something determined by the particular needs of the game, as it gives the game what I think of as a heartbeat. That heartbeat can then be adjusted as needed.
To take two examples, Vampire the Masquerade and Werewolf the Apocalypse are popular TTRPGs originally by White Wolf. As the titles suggest, in the first you play a vampire and in the second you play a werewolf. Several mechanics in the first game have a "per night" heartbeat, because vampires are nocturnal. Several mechanics in the second game have a "per lunar month" heartbeat, because werewolves are linked to the moon. This ties the mechanics to the fantasy in a way that feels very natural.