In order to keep a constant supply of ammonia I changed my shrimp every few weeks. Get ready for some decaying odors.
Make sure that whatever shrimp you get has not been seasoned in any way. The shrimp you want should be grey.
you dont have to excahnge the shrimp as long as theres somethign in tehre decaying itll supply ammonia ive had mine in for 3 weeks and theres still in there and i never had a smell
don't take the shrimp out...just let them decay and in 4-6 weeks you'll have a cycled tank ready to house your first fish...
not sure how big your tank is but the general rule of 1 shrimp for every 25 gallons worked great for me...