To begin to answer your question, it is important to clear the assumption that over-treatment simply relies on frequency of treatments. You can overtreat a patient by incorrect usages of points, herbs, moxibustion, etc. regardless of the frequency.
In acupuncture "overstimulation" would be defined, by myself anyhow, as using points and/or techniques which result in the disruption of the flow of energy within a person. This is similar to creating disruptions by the overcontrol of the breath and/or incorrect postures in Tai Chi, Qi Gong, and/or meditation (i.e. running fire). Really the term should be incorrect stimulation instead of just overstimulation - so using too strong tonifying points/techniques when a patient has an excess pattern, using too dispersive of techniques when a patient has a deficiency pattern, etc. What this will mean to the practitioner and how they watch for it (and correct it) will depend on whether they use strong stimulation and/or directional techniques (not all practitioners do) and what style(s) of acupuncture they are using.
From the patients perspective their responses could range from simply not getting better to feeling "off" after a treatment, experiencing light-headedness, breathing problems, anxiety, etc.
What it comes down to is there are no standards for how long a given condition should take to treat, except for what you learn in school and what you see your masters (if you have one/some) do. So we operate, as all health professionals do, on a series of assumptions drawn from what we have seen work in the past and what we have experienced for ourselves. This may or may not be correct, but if you are getting results people are unlikely to question what they are doing - even if they may get better results or provide the same care to a patient at less cost (i.e. frequency). It is usually when people are getting poor clinical results frequently that they begin to re-evaluate what they are doing.
With regards to frequency of treatments, the following comment of mine to a previous question, found here, describes most of my views on that point (and the link lower in that discussion from an article by Dr. Dharmananda describes the near opposite of everything I said in my comment). While I'm obviously against treating multiple times per week for anything but the most acute of situations (mostly pain cases), I cannot say that someone who chooses to do this is wrong. It's not a matter of right or wrong, it's largely a matter of style and training and also patient needs and restrictions. What most practitioners do, is what they were taught in school and/or what they learned from studying under various practitioners with high rates of positive clinical results. While I disagree on a number of points with Dr. Dharmananda's article, it is an excellent read as is all of his work on their site.