No one really knows (and I'll tell you why later) but the general consensus is that lung capacity declines with age more rapidly in smokers than in non smokers. Quitting smoking does little to restore lost lung function ( so this depends on how much and how long you have smoked) however, after quitting, the decline in lung function tend to parallel that of non smokers. In other words, quitting prevents the faster decline in lung function but you probably won't return to pre smoking days just by virtue of the inherent decline in lung function with age.
The reason why noone knows is that
no studies have ever looked at a presmoker's lungs and measured their (FEV1) (functional expiratory volume) and let him smoke and then measured them after quittting. This is not an ethical study. The only studies compare smokers with non -smokers (who never smoked) and ex- smokers.
Here is a link to a sample study which looked at that type of breakdown among exsmokers, those actively smoking and those who never smoked.
http://ajrccm.atsjournals.org/cgi/content/full/161/4/1264
The surprising thing was that the smokers that continued smoking and exsmokers had similar rates in decline of FEV1 after a five year followup.
I take this study with a grain of salt since it was a small study and they did state that the "exsmokers" may have had greater underlying lung disease which prompted them to quite smoking in the first place. Thus, there may have been what's called selection bias by the clinical participants. Other studies show that the rate of decline in FEV1 is slower in ex smokers vs. persistent smokers.
I suspect the answer is how much did you smoke, underlying genetic susceptibility (those with alpha-1-antitrypsin deficiencies develop emphysema with little to no smoking) and your natural lung capacity before smoking.
If there is any reassurance, I see CAT1 bike racers (near pro level) who used to smoke and are more competative than those who never smoked.
.