According to the Thames River Boats, it costs £17 GBP, but I'm not seeing any mention of times.
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2It's the slowest and nicest way to Kew Gardens. I highly recommend it for one way (go by boat, return by tube). – ugoren Jun 18 '17 at 20:09
While the WPSA website doesn't have a timetable, the Transport for London website does. It looks like it takes about an hour and a half.
(For comparison, it would take about 50 minutes by rail - either the District Line or a train from Waterloo. So not that much slower...)
