From a performance perspective, if it gets very warm for the season - so 13/14 degrees - then the ones I have on (Avon Ice Touring) and other I have read about on here get a bit squirmy. You certainly know when it is time to change them when spring really kicks in. However at this time of year you need to remember that the ground temperature starts to drop (and stay) lower than the air temperature.
In terms of wear mine are now on for their third winter, had them on each year from mid to late November until End March / early April. Tread is down to about 5.5mm on the rears, slightly less on the fronts, which is close to the usefulness limit for winters. I'm led to believe that below 4mm they begin to lose their effectiveness rapidly, I guess because the stippling (?) between the treads that grips snow/ice and aids with warming the rubber is mostly gone. So I think I'll need a new set for next year.
The fronts are less worn than the rears because I have 17" 8J rims on all round, with 225/45/17 tyres on - they are non directional so I've been able to swap the wheels between axles to get the longest life possible.