I've read that if you don't pick them up at the FAI "fast enough" they leave forever.

How long do you have to grab these two Harpers?

This has been my experience...
1. You don't pick them up in the FAI, and go on to complete the Nashkel mines quest,,,when you return to the FAI they will either, A. Not be there, or B. Won't join you.

2. Pick them up and do the Nashkel mines quest,,, if you ever dismiss them, they will walk away and you can't use them the rest of the game.

Hope this helps

That's pretty much the gist of it. Agreed with what Elfbane said ;)

Provided you don't recruit them, they will remain in the Friendly Arm Inn until you start Chapter 2 [entering Nashkel or the vicinity]. If you recruit them and then drop them before speaking with Berrun Ghastkill, they will likely abandon you.

After you've cleared the Nashkel Mines and reported your success, however, they most likely behave just like any other NPCs: You can boot them from the party and they will remain where they are, provided that they're satisfied with your Reputation.