How much should I expect...

Oh, to actually address the original question, it depends on how much fuel is in the tank. If You remove the fuel cap(very important!!) & have the line pinched, You can blow
thru the line back into the tank as you un-pinch the hose, this will break any siphon effect. If you don't remove the cap, even w/a clear vent, it will be dry for about .4secs.,
then you will be getting a petrol facial!!! If the level is high, well, plan to drain it or work on it whilst it's trickling steadily.