Yeah, it depends on when you plan to go. It also depends on what you want to see and do - and where. For instance, France is almost the size of Texas. Your route will be different if you only plan to visit Paris than if you want to spend time in Burgundy or Provence, for instance.
You probably have some ideas of what you want to see and do (otherwise, why Poland and Switzerland and not Portugal and Spain). If it were me, I'd do some research (travel guides, online resources) and get an idea of what places in each country you want to visit. Or maybe you've done that already and haven't given us enough details to offer concrete suggestions. If not, the last thing I'd consider at this point is flights. You might find that you don't have enough time for all the countries on your list. For instance, adding it up, I've probably spent 2-3 months in Italy alone in the past 4-5 years.