A couple of sources putting the life expectancy cost at 10-20 minutes per cigarette.
Smoking for 40 years that would be 5 months at 1 a day or 17 years at 40 a day.
I think that's all consistent with what you said: 2 packs a day and you usually, but by no means always, don't make it to retirement at ~66. Five months is enough that I'd take some care to avoid it, though.