It comes down to the fuel volume in the gascolator, lines, pumps, and float chambers of the Bing carburetors the Rotax 912 uses. Without knowing the precise numbers, we can make an educated guess.
So say it's burning 17 l/hr at cruise, which is 283 CC/min. I don't know the exact number, but say the Bing carb's float chamber holds 50 CC for a total of 100, and the gascolator/filter on the firewall holds another 100 cc of gas, maybe another 50 CCs on the lines between the gascolator and carbs, and another 50 CCs in the lines running to the shutoff valves. Add another 50 CCs for the pumps etc.
So say somewhere around 350 CCs of fuel is available once you shut off the supply, although the pumps won't be able to draw all of it due to the lack of venting at the opposite end at the valves, and it'll depend on the effectiveness of the fuel pump in creating a vacuum between the fuel and valve as it's drawn. Maybe it'll be able to draw 300, which will run the engine for about a minute, maybe a minute and a half at the most. Maybe quite a bit less.
So I would go with somewhere between 30 and 90 seconds.