When an experienced monk meditates how much does their mind wander? I’m wondering if most monks would go 99 pct of time with no mind wandering, just focus on breath.
Case 38 of the Mumonkan An Ox Passes Through a Window 
  
  Case
  
  Goso said, "For example, it's just like an ox passing through a
  laticed window. Her head, horns, and four legs have passed through.
  Why is it that her tail can't pass through?"
  
  Mumon's Commentary
  
  If in regard to this you are able to turn yourself upside down,
  attain one single eye, and utter a turning word, you will be able to
  repay the four obligations above and help the living beings of the
  three realms below. If you are still unable to do this, reflect again
  on the tail; then you will be able to grasp it for the first time.
  
  The Verse:
  
  If it passes through, it will fall into a ditch; 
  If it turns back, it will be destroyed. 
  This tiny little tail- 
  What a strange and marvelous thing it is!


I'll let you in on a little secret - the mind never stops wandering.  Oh sure, the mind of an experienced meditator looks nothing like that of a novice, but to say that it's 99% focused on the breath is a pretty romantic view.  One thing that becomes apparent over the course of one's practice is that our illusions are depthless.  Unless you are the Buddha himself, there will always be imperfections in your awareness.  The only difference between someone who is experienced and someone who is just beginning their practice is the nature of these delusions.

To give you an example, when one first starts their meditative practice, the small mind is pretty much it's own master and it will drag itself around in limitless ways.  There is internal narration, irritation at discomfort, plagues of doubt, and an assortment of other defilements affecting the mind.  The mind of someone who has been sitting consistently for, say, ten years is much more spacious.  One's internal narration almost completely disappears.  Discomfort more or less vanishes.  Doubt has mostly been cured.  Those defilements that remain are much, much more subtle.  Most of them are completely beyond the realm of language.  Here, imperfections in awareness are dealt with in a way comparable to a master craftsman who knows how to apply his file just so to get the wood to respond in the way he wants.  Just how he could never describe the feel he uses to apply his craft, the skilled meditator simply cannot vocalize the feel they have for keeping their mind applied to a counter-point sign, for instance.  

More to the point, the deeper one engages with emptiness, the more likely they are to discover a veritable ecology of subliminal mental obstacles, habitual patterns, and other phenomenon arising from the personal unconscious (e.g. those issuing from the alaya vijnana or store-house consciousness).  These subtle, unconscious phenomenon merely "perfume" the experience of the novice.  Their presence is so pervasive - like an odor from an old, soiled couch that lingers in a room - that they are mistaken as being fundamental to the nature of experience.  For the the experienced practitioner, on the other hand, these phenomenon are seen for what they are and become the fodder for much of his practice.  They are also the very things that will give rise to most of the subtle distraction he experiences in meditation.

Off the cushion, the experience is mostly the same albeit it is much, much more difficult to maintain a deep level of mindfulness with so much going on in the world.  The mind is far less stable off the cushion than it is on it.  Because of that, any disruption makes  a much larger splash.  Because there is so much going on, it is also much harder to really see the much more subtle defilements (like those arising from the alaya).  One might think that they are free from disturbance, but the truth of the matter is that their mind isn't quiet enough to see the torrents raging just underneath the surface of consciousness.

In short, no matter how experienced you are, there's always that tiny bit of tail.