What does “snooze factor eleven" mean?
Snooze is to sleep. By extension it implies extreme boredom.
Factor is a measure of something, and is often used along with numbers as in "sun protection factor" as a measure of sunscreen effectiveness, "warp factor" in Star Trek, numbered blood factors.
So "Snooze Factor [something]" would make it understandable as meaning it was very boring and if that "[something]" was a number it would fit a pattern recognised from elsewhere, even though that pattern works differently in different cases.
But what number to use?
Well, in the 1984 comedy "This is Spinal Tap" one character boasted about his custom-made amplifiers:
https://www.youtube.com/watch?v=4xgx4k83zzc
Nigel Tufnel: The numbers all go to eleven. Look, right across the board, eleven, eleven, eleven and...
Marty DiBergi: Oh, I see. And most amps go up to ten?
Nigel Tufnel: Exactly.
Marty DiBergi: Does that mean it's louder? Is it any louder?
Nigel Tufnel: Well, it's one louder, isn't it? It's not ten. You see, most blokes, you know, will be playing at ten. You're on ten here, all the way up, all the way up, all the way up, you're on ten on your guitar. Where can you go from there? Where?
Marty DiBergi: I don't know.
Nigel Tufnel: Nowhere. Exactly. What we do is, if we need that extra push over the cliff, you know what we do?
Marty DiBergi: Put it up to eleven.
Nigel Tufnel: Eleven. Exactly. One louder.
Marty DiBergi: Why don't you just make ten louder and make ten be the top number and make that a little louder?
Nigel Tufnel: [pause] These go to eleven.
This film was only moderately successful when it came out, but gathered quite a cult following subsequently, and so for people from Anglo-phonic countries of a certain age there is a running joke that the highest possible number for anything is 11.
So to be "snooze factor eleven" isn't just to be boring, but to pull out all the stops and really go all out at being as boring as possible.
Is 'snooze factor' a common English word?
Remember in the era of Star Wars, it was very common to riff on "Start Wars"? So, headline writers and article writers would have "Xyz Wars" to the point of overuse.
Today in all English-speaking regions "The X-Factor" is a huge TV show franchize. It is common to riff on the name .. so, sleeze factor, etc. Here "snooze factor".
It is especially common in writing about media (as here), writing about other TV shows (as here), and writing originating from media critics (as here).
"Why should it be “Eleven” ..."
"Eleven", "turn to eleven", "it goes to eleven" etc - from This is Spinal Tap - is, along with let's say "Don't Panic" (from Hitchhiker's Guide) or "We're not worthy!" (from Wayne's World) one of the most famous comic references in English in recent times. 100% of native speakers are familiar with the joke.