It depends the crystals can hold about 36 and its usually stored before hand like I said... It really does depend on the tpe of elementals using it because some are longer or harder than others, Water, fire, and darkness would be the fastest, and divine, light, and wind would be the hardest and longest.
Fo the shortest, water, it would take 30 minutes. But for a divine, the longest, it would take quite a while.