Depends on your smoker type/model, chamber temp, finished internal temp, cross-sectional density (thickness), how much ratio of stuffing vs meat, type(s) of stuffing, etc.
If you have a 1.5lb loin stuffed with0.75lb of goodies, you now have 2.25lbs, but you could figure approx. 1.25-1.50hrs/lb to reach 145* I/T (minimum recommended temp for USDA inspected pork) with a 225* smoke chamber temp, unless it's bulked-up quite a bit from stuffing it...then you could be looking at >1.5hrs/lb. I've never smoked one that small, but chicken leg and especially breast quarters take even longer for me...higher finished temp (min 165*), yes, but quite a bit longer.
BTW, is this a tenderloin, or the whole loin? Tenderloin is much smaller diameter and cross-sectional density to begin with, so would cook far quicker.
Main thing is, cook to temp, not time...and time is difficult to estimate...all smokers act differently than the next, and weather related factors will effect time also.
Eric