20l/min is quoted by the manufacturer (I looked on the side of mine).
Realistically I think you could get by on 12-15l/min.

But then, what's the point of having a megaflo? You might as well just save
your cash and get a combi, which is half the price (or less) than a system
boiler/megaflo combination. If your total supply is much less than 30lpm,
you might find it is a waste of money.

Christian.