When you realize that you have not been thinking the mantra for a while you simply start thinking it again. This lasts about fifteen minutes. This is how we also think the mantra. The position should be sitting upright but relaxed. Breathing typically becomes flat during meditation while the breath frequency goes down, occasional deep breaths may occur.
Meditation regulates itself.
Disturbing noises or thoughts are registered but ignored. One just starts thinking the mantra again. It may happen that the mantra is not thought once during a session. This is a hint on massive distress or distractions and can be ignored as long as it happens only occasionally. Flat breath, feeling of deep relaxation and ease.