My experience is a little different than those posted so far. I know it's not the "norm", just thought I'd share a different scenerio.
I used cured LR to cycle my tank (almost 1 lb. per gallon of water, as you can see in my signature). My tank DID cycle, showing spikes of ammonia, nitrites, then nitrates. Each died off just like they should have. This happened in 10 days. I did not have "significant" die-off from my LR, but enough to create a cycle.
BTW, my ammonia, nitrites, and nitrates have been excellent since my cycle ended.
HTH,
~D~