I would hit the girls with pollen between days 15 and 20 of flower, depending on how many calyxes they've packed on so far. You need enough calyxes to get a decent quantity of beans. All calyxes that grow after the ones you've seeded will just be empty as usual. Pollen can stay viable for a couple of weeks afterward, but for the most part only the calyxes that you pollinate will grow beans (with a few strays here and there).