So for anyone who has played the game, does this mean that Allies are used quickly and discarded, never to be seen or heard from again? Or do they tend to hang around waiting for just the right moment to help out, which may mean that they are in play for most of the game?
It depends by what you draw. You can have up to three Allies in play at once, so if you're really flooded with them, you could like to use them quickly in order to replace them with those you've still in hand. Otherwise, you'll usually prefer to activate them when it makes more sense.
More often that not, you'll pay only the better ones, discarding the others to activate the special skills of your Heroes (many of them require to discard a resource card from your hand), especially since playing Allies needs precious Plot Points (especially expensive for the Allies non affiliated with your team).
Your style of play ("aggressive" or "conservative") will also be important, of course.