Winter tyres should be used from November until end of March, when the temps are usually low and the weather wet, so about four months. This is quite a long time. I think though that you still miss the point and are thinking about snow.
If you choose to ignore all the evidence for the safety benefits of fitting winter tyres and if you choose to ignore all the personal experiences of people on here using winter tyres then so be it. What surprises me is that you, being in the profession you are, do not recommend the fitting of winter tyres to your customers because of the short winter spell we in the UK experience, ie snow. You are giving very bad advice. If I was you I would, after reviewing the evidence and trying a set of tyres, pro-actively sell the benefits of fitting winter tyres to your customers.
Furthermore, showing pics of tatty cars really doesn't inspire me to visit your garage especially if you cant grasp what is a winter tyre/summer tyre.
The mind boggles.
Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow. Winter=wet, slush, ice, not just snow.