This is a great question. But I think you need to clarify if you are referring to a steady rate of exercise (such as walking or running at the same speed or biking at the same RPM’s) or HIIT workouts (such as Crossfit type training). In the case of the first one, your rate of calories burned will eventually drop because you will become more efficient and you will require less energy to do the work. This will lead to a plateau. In HIIT I believe the calories burned will increase for two reasons:
-You will have to increase your weight and/or repetitions when doing the exercise because you will become used to the same weight and reps and in order to keep the same intensity level you will have to adjust these two variables.
-Your BF% will drop, which means your lean mass will increase so you will be burning more calories doing the same work. More muscle tissue means higher metabolism which equals to more calories burned.