According to my experience, meat continues to take on smoke for as long as you want to add it however, it does seem to get the most benefit when smoke is added at the very beginning.. i.e. when the meat is at room temperature or slightly colder.
Having said that, I recommend continuing to add smoke for about half of the estimated cook time. If you are smoking a whole chicken and the estimated cook time is 4 hours then just add smoke for 2 hours.
If you are doing spare ribs and the estimated cook time is 6 hours then just add smoke for 3 hours. This formula seems to work for me in every occasion.
There is also nothing wrong with adding smoke throughout the entire cook.. after all, if you were using a wood smoker, it would be getting smoke from start to finish and nothing tastes better than meat cooked in an all wood smoker (my opinion of course).
If you need a chart of estimated cook times, I have one
here.