How often you do it depends on your particular personality and, sometimes depends on what is occurring in the practice.
How do I know when I am doing it too little? When there's a lot of restlessness and agitation, speediness perhaps, feeling overwhelmed by the thoughts. For example, you're trying to contemplate but there's so much agitation; so you may have to do some breath meditation at those times.
Sometimes, if people are able to really let go of the thinking process in the walking, they may not need to do as much breath meditation. But, if this is not working very well, then we may need to watch the breath and calm the mind. So usually when there is a lot of restlessness in the mind or the feeling of mental exhaustion, it may be, if we've been working very hard in the reflections, it may be that we need to refresh the mind with just practicing letting go of thought for a while, letting go of reflection, calming the mind sufficiently so that we can then have the energy to begin to contemplate again.