Here's what the thresholds are for various match length notes in the product descriptions.

[awesomenessofme1]

I wasn't able to find anything about this with a quick search, so I decided to test and compile it for myself.

"Fans expect important matches to be quite lengthy"
An important match is one that contains at least one Star or Major Star.
9 or less: "The match was far too short for an important match in the eyes of our audience."
10-14: "The match was too short for an important match in the eyes of our audience."
15-19: "The match was a little too short for an important match in the eyes of our audience."
20 or more: No note.

"Lengthy matches will tend to lose the audience"
"Fans expect matches to be short and will get bored if they go too long"
"Fans tend to have a short attention span, so longer matches will not do well"
As far as I can tell, these are all the same. It's possible the actual effects are different, but the thresholds are the same.
15 or less: No note.
16-20: "The match was a little too long given our audience."
21-25: "The match was too long given our audience."
26 or more: "The match was far too long given our audience."

"Longer matches will not do well in this environment"
25 or less: No note.
26-30: "The match was a little too long given our audience."
31-39: "The match was too long given our audience."
40 or more: "The match was far too long given our audience."