Is there a benchmark that one can use when consulting a Rav to discuss the possibility that his observance of certain laws needs to be considered as though he has an illness?
The minimum level of illness which modifies observance of certain laws is what is called in the laws of Shabbos “choleh kol gufo” (illness of the whole body) which is dealt with in Shemiras Shabbos Kehilchoso Chapter 33. The first two criteria are:
Someone who has to go to bed because of his illness
Someone with a high temperature which would keep him at home.
See this section for more details.