Acetone is produced naturally by your body, so injecting small quantities probably wouldn't have much effect. I can't find any data about the fatal dose in humans by injection, but the fatal dose (LD50) in mice by ingestion is said to be 3 grams per kilogram of body weight. If we can scale that to humans - and I have no idea whether we can do that reliably - we would be talking about something in the region of 200 - 300 grams as a dose that is likely to be fatal. You would probably want to up it a bit, say to 500 grams, to be on the safe side. Given the density of acetone, that corresponds to a volume of around 650 millilitres, which is more than a pint. That's a lot to inject. (You would probably have to put yourself on a drip.) It doesn't seem very practical to me.