The way I see it is that the power within grayskull is sentient and it allows he-man or she-ra borrow part of it's power for it's own protection. So I don't think there's a time limit but the pog would "know" when it's not needed per se at which point Adam or Adora would revert to their original selves.
As for the swords, I think they should be viewed at as keys or conduits which channel the power. There's no real power in the sword. That's also why m@a was able to construct a techno version and the need for another sword on Denibria.