I have been reading about how peat moss in the garden is one of the best things you can add to it. I was curious if anybody could share their experiences on how long the stuff lasts. do you have to replace it every year?

Also has anybody ever used coconut coir? supposedly it is the environmental friendly substitute for peat moss

About a year, but a lot depends on how much you put in, and how much you till it in with the soil.

http://www.dirtdoctor.com/organic/garden/view_question/id/1800/
read the whole page, esp the last portion...