From what I've seen they last about 15 minutes. Then they go away and you have to resummon. They do not use any extra crystals other than what it took to summon them.
15 minutes, and they fade away and you have to resummon. no ther costs. if you die or crash, they also go away, and have ot be resummoned.
everyone that had cubics have been complaining about the 15 minute timer. why the @#$% is it 15 minutes and not 20? puts cubics out of sync with buffs and, if you have them, summons.
still haven't had a concise answer from NCSoft about this either...