Well temp outside isn't too bad, freezing here.
If you load up a full chimney and dump it in, bring it to temp, not sure what you are doing for wood.
If just adding for flavor and using coals for a heat source then I would say the first batch will run you about an hour but much of that will be coming up to and regulating temp, after that if you add about another 3/4 full chimney of hot coals then you should be able to get about 2 hours out of it, maybe a bit less.
Again this is going on coal only, I burn a LOT of wood and coals are just for starting out and boosting temps but use mostly oak for a heat source.
As the ash builds up in the bottom of your firebox it will insulate and actually help to keep the temps up a LOT.
If you ever pull ash out in the middle of a good smoke you will notice it is glowing red in the center.
So you should get to a point after a few hours where about a chimney full will burn for about 2 hours nice and solid before you should have to add more, but then when adding more you could go a little lighter (half a chimney full) due to the insulated heat in the ashes.
Hope that this helps.