After 20 mins:
Blood pressure and pulse rate return to normal

Eight hours:
Poisonous nicotine and carbon monoxide levels reduce by half, and oxygen levels return to normal

24 hours:
Carbon monoxide totally eliminated from the body

48 hours:
No nicotine left in body - senses of taste and smell greatly improved

72 hours:
Breathing has become easier as bronchial tubes start to relax

2-12 weeks:
Circulation improving

3-9 months:
Lung capacity improves by up to 10%

One year:
Risk of heart attack halved

Ten years:
Lung cancer risk halved

15 Years:
Same chance of a heart attack as lifetime non-smoker

Quelle: BBC :)