I was changing mine everytime the smell went away. I figured that damsels would have created a constant supply of ammonia, so I imitated it w/shrimp.
However, my ammonia level would not raise again after 3 weeks and the addition of 30lbs of LR(some uncured) so I stopped adding shrimp. This may have been a mistake, but there was allot of die off on some of the LR and my ammonia level had reached 9-10ppm at one point. The level all the references I checked said it should.
As I said in a very recent post, I am still waiting for my nitrites to lower and am in the 5th week.