The first day
Within about 20 minutes of your last cigarette, your heart rate and blood pressure start dropping back toward normal. Within around 12 hours, the carbon monoxide in your blood clears and your oxygen levels recover.
The heart recovers fast
Excess coronary heart disease risk falls quickly, roughly halving in the first year or two, and approaches that of a never-smoker within about 15 years. Stroke risk returns close to never-smoker levels within about 5 to 10 years.
Cancer takes longer, but it does fall
Lung cancer risk keeps declining for decades (though for heavy long-term smokers it stays somewhat elevated). Mouth, throat and oesophageal cancer risk approaches baseline over about 10 to 20 years. Bladder is the stubborn one: it stays elevated, around 45% above a never-smoker, even decades out.
Timing matters more than people think
Quit before about 40 and you avoid roughly 90% of the long-term risk of dying early from smoking. Earlier is dramatically better, but it is genuinely never not worth it. The curve bends down the day you stop.