When you still have the metal pinch when it's hot it's defiantly not broken in yet. But you will notice that the engine wants to go faster and faster on a point, around then the worst pinch is gone and you can start leaning the engine. Then i always keep driving it fairly rich until it get's a stable tune and doesn't richen itself while on the track.

The metal pinch when cold could stay for a long time if you have a good engine.
For example, one of my current JP engines still has tremendous metal pinch after 5 liter of fuel. But when it's warm the metal feel is gone and it is running perfectly.