I would say that on a stock or stage 1 engine lugging would be under 2500 rpm and more then 1/4 throttle, but it is a general statement as you would have to define both rpm and load. Obviously you can let the thing idle without lugging it. LOL.
You cant put a RPM number on when an enging will lug. There are too many other variables, most importantly of which is load. It's a seat of the pants feeling. If the engine feels like it's lugging,....it is.