Please use the search engine. This has been answered in the past.
We, and everyone else, is unsure exactly what spice contains in terms of research chemicals. Therefore, *how* effective it is, and in what dosages, we cannot tell you, as we don't know which synthetic it is.
Further, the mixture they use is probably covered in toxic bullshit, I would not put it past those fuckers. If you insist on using RCs, at least acquire the safest route - a powdered version of the actual drug... not some even MORE unknown version that is a financial rip off of epic proportions. For information on that, read through the mega synthetic thread here in CD. It's dense, there is a lot to be learned.
/rant.