Royal Engineers black book
All that I can remember is a "hasty" formula in said black book.
You can forget finesse because the pipe will "gloop" and effectively run half-full and will have very little pressure head to drive it.
The book said 11 UK gallons/min for a 3" armoured hose and 22 UK gallons/min for a 4" armoured hose. Call that 50 litres/min and 100litres/min respectively.
There are classical formulae and "deliberate" formulae as the Royal Engineers say but none of these are applicable to the sort of jury rig an old-fashioned gravity feed might suggest.
Nevertheless a proprietary 2" gravity feeder at RAF Gan could do 150 litres/min.
Therefore you are probably looking at a spread of 20mins-60mins under gravity, limited by frothing and glooping.