I would take the boat (or monorail) from Poly to MK, and then transfer to the boat to FW. I'd give myself at least an hour, though it should take less time because A) I hate to be late for anything and B) FW is a cool place to just explore and hang out.
Of course, if your smarter (and less frugal than me), you could simply take a taxi, which would be much quicker. It's probably a 10 minute ride, but I'd still leave at least 30 minutes before.