AFAIK the 124 tank capacity is 72 litres (approx 19 US gals), and the fuel warning is meant to come on at 12 litres (approx 3 US gals). This is roughly how it functions on my 260E, as filling up when the light comes on usually comes to 60 litres.
The 12 liter fuel warning level has also been mentioned on other forums. Also, on some mercs, I know that there is a 'final warning' fuel warning, where the light flashes instead of simply remaining lit. I have nevered dared venture into this terrority on my own car, though this info has been related to me by other owners, together with another story from a W202 owner who's warning light wasn't functioning and he ran out of fuel. Twice
P.