I'm currently playing GURPS on a play-by-post system, and one of my characters has an ability ("Luck", allows me to reroll an attack or ability check and take the better of the two results) that can be used once per hour as time passes at the table.
The problem is that in a play-by-post environment, there will often be only one or two "moves" per day for most players and the GM, because of needing to wait for other players to post their moves and the GM to respond.
What's a reasonable compromise value on how often I can use Luck in this kind of game? Clearly, an hour of player time isn't right in this case, else I could use it on virtually every roll. But an hour of in-game time isn't the intent of the rule; in combat, one might get to use Luck twice or even three times in a battle, but in uneventful travel one might be able to use it only every few game days — or weeks.