+1 Alexander. I would add the following observation: The book is about the transformation a character is going through, what is commonly called an "arc" but is in essence a change. Sometimes a literal change, like "coming of age" stories, or romances (becoming a lover or spouse), or ascension (becoming a leader), or stories of a loved one dying (which changes us, perhaps into an orphan or widow or widower, or the loss of a friend or teacher changes us in some way). In other stories it is just a change of personality: From gang banger to priest, from frat boy to soldier, from fearful to courageous, from humble hobbit to heroic champion. From kid on a moisture farm to Jedi Knight.
Time spent inside a character's head is fine, but if it is long, it stalls the story. They do need to process experiences and new information, but walking in mental circles just gets boring. If the thoughts are not fitting pieces together and coming to new conclusions, then the reader will get bored. They expect us to skip that part and show them the thoughts and scenes and experiences that advance the change, whatever it may be.
If the time inside the head is not changing the character by solving puzzles, making realizations, reaching conclusions or inventing plans of action (that they intend to carry out), if they are just wondering and wandering and accomplishing nothing, the space is wasted.
Humor has its place and is welcome, but is seldom welcomed in a long form, pages long, and that will just stall the story. Readers will give you some rope and read for awhile when they aren't sure what the point of a scene may be, but they DO expect there to be a payoff at the end. If the payoff [a character changing moment] is not evident, or seems small and incremental compared to the length of the scene, they will be dissatisfied with the writing.
If that is a recurrent theme, they will start skipping to the end of the scenes to see if there is any payoff. And finally they will put the book down, as too much filler and not enough plot, or suspense, or events of interest.
Character change almost always requires an outside catalytic interaction, something that forces learning or philosophical re-evaluation, experiences with the outside world. It almost never occurs by long introspection, most minds have long ago achieved equilibrium of beliefs and habits and performing the duties and rituals of daily life. That equilibrium must be disrupted, somehow, in order for change to occur. The disruption is usually unexpected, and requires dealing with the outside world in new ways.
Writing internal dialogue because it is fun might be distracting you from the damage it is doing to your story as an irrelevant roadblock that breaks the expectations of readers.
I would say it becomes "too much" when the thoughts stray into areas that will have no impact or influence on the character's actions, or other characters, or the plot. Then you are wasting the reader's time with irrelevancies.