Normally about an hour to an hour and a half or so. Pretty much matches the time to get a 1.25-1.5" thick ribeye to medium. Depends on how thick the onion chunks are, and what temp the bbq or smoker is running. On the Weber bbq, which runs hotter than my smoker, about 45 min most of the time. Also, don't use thick cut bacon. Takes longer and isn't as pliable for wrapping. Tears easier. Normal Oscar Meyer bacon works good, or even a good store brand, just not Morrel brand. That bacon isn't very good. Meijer brand even works better.
These do not work out well on a gas grill. Normally causes a nice fire, over-done bacon and under-cooked onion inside.
As we all know every smoker/pit cooks differently so experimenting is key as well as half the fun. I am going to try these next weekend. Have you thought about using foil? One could wrap initially and then crisp the bacon over direct heat/smoke and then encapsulate that bacon flavor profile directly into the onion by wrapping them in foil after semi-crisping/cooking the bacon. May I assume the onions are slightly pealed?