Most of the answers on here are adequate, however, I do not feel that they get straight to the point.
How much detail is too much?
I've read a few amateur stories online and sometimes the author will include so much detail that the reader gets a little bored or distracted and forgets what the actual story is talking about.
You have answered your own question:
Q: How much detail is too much?
A: When the reader gets a little bored or distracted and forgets what the actual story is talking about.
The details in a "fully expressed medium of text" (that is, a sentence, paragraph, article, book, or voluminous/episodic series) greatly influence attention. The center of attention is the focus.
Superfluous details are details that divert attention away from the focus. Consider the following:
I was dropped off at the usual bus stop that I attend every morning. It is a short stretch of concrete road with green signs, and four houses. There are two houses on each side. Each house is white. The lawns varied in shades of green and brown (it was early fall). When the bus finally arrived, I hopped on. Silence was in the cold air of the bus as students stared at their phones and closed their minds off from the real world to escape into the fantasy of the internet.
The focus of this paragraph is getting on the bus, but yet, all the detail is focusing on the surrounding environment of the bus stop, before abruptly transitioning to hopping on the bus which appeared out of no where.
Necessary details are consistent with the focus. Consider the following:
I came to this large, two-story house with overarching balconies and elegant columns. It was old—very old. I burst open the dusty, wooden double-doors that entered into the house. The room was dark, and had a musty, century-old odor to it. The black and white checkered tile pattern on the floor was heavily worn and coarse. The chandelier dangled askew. This was, no doubt, a dance hall, which told the story of a time long forgotten, when people twirled and danced to a merry tune.
The details in this paragraph dictate that the focus is an old house which had many ballroom-style dances. Every detail describes the age of this house, and the purpose is to give imagery of what the house looks like, and what the ambient atmosphere projects. Without these details, this would be a dull paragraph, and the actual lack of detail would bore and distract.
Detail is of course only one element of influence for the audience's attention, but it is clearly displayed that it does have a great impact on attention. The detail determines the focus,
so how much detail is too much?
When the details shift attention away from the center of attention.