At 23.5 litres. Ran out this morning, put 5 liters into the bone dry tank, rode 7 kilometers to the servo, current consumption is 14km/l (grrrr until I take it BACK to the dealer next week), filled to the top with exactly 18 liters.
I guess being a plastic tank there will be some variation, I'd imagine 23l would be a rock solid safe estimate.