Hi Eddie
I'm currently researching this myself as I am looking to start up in about 2 months time. From my research so far and marketing I have done in an unrelated industry using this method if you are delivering yourself you should expect a return of 0.25% - 0.40%, so on average 2 to 3 per thousand. However it also depends on where, when and how often you deliver. You will have a better response if you cover the same area on a regluar basis, say every 6 to 8 weeks. You will also have a better return if you include some kind of offer.
You also need a good leaflet, try a google search for carpet cleaners leaflets you should come up with a search for DP printers, lots of samples on there that you can download to use for insperation. Spoke to Mark there today, very helpful with advice etc.
Hope this helps.
Richard