Something like 19 useable litres, or just over four gallons in old money.
On mine the range varies a lot, my Alps trip highlighted this as I bothered to keep records.
At worst two-up loaded at 90mph it will averaged as little as 42 mpg giving a range of about 160 miles.
With mountain switchbacks keeping the speeds down and it is far better and on one two-up ride it averaged 52mpg - and a range of about 210 miles.
I actually fare worse when solo, probably through not being so smooth, but generally get somewhere between 46 and 48 mpg.
I tend to just fill up shortly after when the light comes on, which is between 140 and 160 miles for me.