All pollen requires some moisture to activate it’s biochemical reactions for growth. (Pollen grains grow in a similar way to how a seed grows, by shooting out something akin to a tap root, and transferring the genetic material to the inside of the female pistil tube, then it’s carried to the ovary and impregnation takes place.) The key is that the less moisture available the more viable pollen you will have. Keeping it cold and dark will also help. Pollen can be used years later as long as it’s super dry and tossed in a freezer.
When the majority of pollen sacks have opened up, or the plant starts dying, which happens just a couple weeks before the flowers of the same strain would be finished maturing (few translucent trichomes and mostly milky, or amber, trichomes).
I usually wait for the majority of the sacks to open. Occasionally I’ve had to take them early, because of the health of the plant, but I’ve usually still gotten a decent amount of pollen by drying all of it quickly and shaking it in the garbage bag. I usually hold all of the stems together, put the tops into the bag and wrap the bag edge around the stems. Then I just beat up the bag by swinging it, lightly, against a wall. Shake the bag so all the pollen falls to the bottom and mulch the plants. Collect pollen from the bag into a container, pour some desiccant beats in with it, label it and toss it in the fridge. Free desiccant beads are found by cutting open any of that little white satchels you find in a zillion products, that usually say something along the lines of “Don’t eat me” on them.