How much San Pedro cactus to experience effects?

Hello, I recently bought three 6 inch San Pedro cactus cuttings (mostly to plant and grow, but I want to experience the effects once), and was wondering, roughly how much of the plant (how thick of a slice, etc) will I have to ingest for a medium strength effect? I do understand I have to take the needles off, that much is clear to me, and I have been searching on Erowid but I figured I'd post here just for kicks.