On page 18 of SoHk, bottom of Power Skills:
Casting a spell drains 1 Exh/2rnds.
Concentration drains 1 Exh/6rnds.
Paraphrased:
"If a character desires to maintain concentration for an extended time, he should develop meditation skill which allows you to concentrate without spending exhaustion points."
My group uses Meditation a lot. It's worth a lot of bonuses if you have time and a suitable environment for it.
Also, Elves require Meditation and therefore Meditation skill checks to enter or leave their trance state.. This is pretty funny when they have programmed their trance state for a few hours, but they need to jump awake at once. Sure they get a +25 to Meditation, but they still mess up a roll sometimes.
"OK, we fought off the Orc surprise attack thanks to the Ranger keeping watch all night. Now somebody smack the Elf (+25 to leave trance) and let's break camp."