it goes much much quicker if you clear off a wide section of cliff so that you have more magma "waterfall" space.If you have vulnerable areas the attacks will skyrocket untill the magma is finished its work.
Even spillage from a bridge is enough to redline the chasm eventually. And, personally, I've never seen any increase in attacks from flooding the chasm with magma.[ August 28, 2007: Message edited by: Shadowlord ]