More importantly you need to know the strength of the substance, and what exactly it is.
"Fantasy" is one of those slang terms that we try and discourage here at BL because it doesn't really mean anything, or rather it could mean a lot of very different things.
Is it GHB, GBL, or 1,4? Ask the person selling it to you, and if they don't know, or haven't heard of any of those terms, then I'd say walk away and go somewhere else. These are different substances, often sold under the name of "Fantasy" but they each have their own dosage ratios etc.
Chances are it will be 1,4, as the other two are extremely hard to get a hold of in Australia right now. If this is the case you will then have to ask whetheror not it has been diluted, and if so to what ratio. Once again if your dealer shrugs and says "Dunno" it is time to go elsewhere.
This is incredibly important because if it is 1,4 diluted at the "usual" ratio of 125grams to a litre, a typical dose could be between 11ml and 20ml. However if it is undiluted "pure" 1,4 the dose you need might be somewhere between 2 and 3ml. Taking 20ml of pure will guaranteed put the unsuspecting into casualty in a coma.
If you can not be sure of what the strength of the 1,4 is you shouldn't take it, or at the very least you should use extreme caution when trying it.