I reset the bennies at the end of a full adventure, but that can be a couple of months long.
Although that's the only fixed replenishing moment, I try to make sure to reward good Roleplaying, or use of hindrances during the adventure.
Occasionally I ask players for an Interlude, rewarded with a benny, to get a bit of back story. Like when I introduced the school bully in MHC, I asked how each of the characters knew the bully, any past history...