The GURPS "Luck" advantage gives you rerolls of the dice for (something like) every hour of play. This assumes people sitting around a table playing.
How do you handle this in a PbP?
One could say that one knows about how much gets done in an hour of play, and when that much has happened the luck is recharged. One could trust the players to be fair about this, or the GM could declare when the luck has recharged. (That means extra keeping track for the GM, though, and could potentially also lead to gratuitous arguments.) Or, one could say that it recharges after a certain number of posts.
There are some genres (notably GURPS Action, which is really a sort of meta-genre) that really depend on Luck.
What have y'all done?