Buddy, don't go by time, rather temperature.
Last week, I grilled 1, 4lb chicken on my propane grill. I grilled it, something like 300 - 325 degrees. It took dang near 4.5 hours to complete to get to 180 degrees.
Yesterday, I smoked a 4 lb chicken in my MES smoker. I grilled it at 275 degrees. It took 4 hours to get to 175 degrees.
It's really difficult to judge the time it'll take. But the more food that's in your smoker, the more time it'll take to completion.