hi mate,
I have found variations in how long it takes rats to bolt, i would presume this is how big the chambers are etc below ground, also i suppose how long the rats can tolerate the fumes before they leg it.....( you should here them start to cough hahaha)
I tend to keep the smoker going for a good while until the dogs start to loose intrest in the area, then move on also i move from hole to hole just to make sure that its getting filled with smoke.
JDF