I think that adamhornsby has/is looking at getting an ironing system which has a 350ml water tank and the water is converted to steam at the rate of 40g/min = 40ml/min and wants to know how long the tank will last. If that is the case then Hypognosis is correct at 8.75 mins BUT it is unlikely that the steam will be used continuously so, in reality, it will last longer than that in terms of clock-on-the-wall time ie, the tank might need to refilled say every 30 mins, depending on how continuous the use is.