From a monetary policy point of view the question can be rephrased as: What is the level of growth at which the economy can operate without generating higher inflation? It is always difficult to determine the speed limits of an economy, but in times of change this is even more challenging. One development makes it particularly hard.
That is the move from an industrial to a post-industrial society. In the 1950s, 30% of the US labour force was employed in manufacturing. Now the number is 10%. We have moved to a service oriented society. In the service industry, capacity utilisation cannot be measured by counting idle capacity in factories. Besides, productivity growth can only be estimated in an imprecise way.