My Kelpie started coughing Friday morning, as we were going away for the weekend he went to stay with my mum so as not to pass it on to any other dogs.  My Setters havent shown any sign of it and mum is going to hold onto poor Jack for a few more days in the hope that they wont catch it.  He had the real bad retching cough Friday but Saturday he improved and now he only coughs when excited and then its only a small cough. 

My question is, how long do dogs incubate it before showing signs?  and how long after they stop coughing are they no longer contagious ?

10 days give or take a couple but they remain infectious for several weeks. It could be the strain you Kelpie is suffering from has already been contacted by the setters and they will not get it.

The Setter boys had it quite bad as pups (one was 12 weeks the other was 9 months) we had bright green snot and everything   They havent had it since though .....

Unfortunately Kennel Cough is like the human cold it is caused of dozens of different viruses, we and the dogs become immune to each strain after we have contact with it whether of not we have been ill providing the infection is enough to stimulate our or our dogs bodies to produce antibodies we will develop a resistance to that particular infection and although we may be a little unwell we do not suffer the full blown effect if and when we encounter that particular virus again.

As people and dogs get older they become more and more protected against the sort of virus that causes upper repository infections. It is not possible to treat the virus that causes the problem in the first place but secondary infection can be improved with antibiotics and the symptoms relieved with cough mixture.