We have 25 PDC students who are using the rocket stove to heat their hot showers at the moment.
I just asked the room and they estimate it takes 20 minutes of burning to get the first 5 minute hot shower then 5 to 10 minutes of burn time shower after that. They are collecting a small barrow load of sticks each day. The ambient temperature here at the moment is 16 to 25 degrees celcius, but the water is coming from a deep dam that is at around 12c.
We have a Mediteranean to cool temperate climate here with minimums as low as -12c and maximums in summer of +38c we use the shower all year round. http://milkwood.net/2009/07/12/the_rocket_powered_shower/
Clearly there is more we could do to make it more efficient such as:
[li]increasing the size of the heat exchanger - a lot of hot gas escapes from the to[/li]
[li]raising the storage tank higher than the heat exchanger[/li]
[li]insulating the heat exchanger[/li]
but it works and it works a heck of a lot better than any other wood fired water heater we've tried