I used almost a complete large can, 3.78 litres.
Wow, I would have expected more. I think my helmet alone is at 0.3 litres already and will go up to about half a litre until I'm done with it. So I'm estimating that I'll need more like five litres for the whole costume.
Anyways, keep in mind that resin doesn't have an infinite shelf life, especially once it's open and you can always buy more later.