The shutter of a Canon 5D MK II has an estimated lifespan of 150,000 actuation. When the actuation count reaches this figure or higher, it may be necessary to replace the shutter - something that can be quite expensive. Fortunately, if the camera is in good condition and functioning well, you may still be able to use it beyond its limit. However, you should keep your eye on the shutter count to monitor any changes in image quality or autofocus performance - they could indicate that it's time to consider replacing the shutter or upgrading to a newer model.