Check the spec of your plugs, my NGK plugs only need 6 seconds, winter or summer, in fact the seasons shouldn't matter, a timer relay doesn't know now cold it is outside! Plugs will soon burn out if you give them too long energised, then you go down the slippery slope of thinking they need more and more time when in fact they are getting more and more knackered.
Ive had mine on a switch for the past ten years, I think my relay is only 70 amp, check your plugs specs.