Hi,
If the employee stretch and work beyond office hours due to work requirement , smoking breaks can be entertained.
However if employees are going in group for smoking and waste time in chatting the same should be monitored and restricted.
My suggestion is morning and evening 1 break with duration of 5-10 minutes will be ideal provided the employee completes the work even after office hours.
If you have Door Access Control System you can assess the actual time break.
In the better interest of the health of employees you may plan for some awareness program as to how smoking is dangerous.