How long is a run-on sentence?

There's no set length for a run-on sentence. It's not about the number of words but about the grammar!

Here's the key:

* Run-on sentences lack proper punctuation and/or conjunctions to separate independent clauses.

Example:

* Correct: The dog barked at the mailman, and the mailman ran away. (Uses a comma and conjunction "and" to separate independent clauses)

* Run-on: The dog barked at the mailman the mailman ran away. (No punctuation or conjunction)

So, a run-on sentence can be short or long, but the issue is the lack of proper grammatical structure to connect the independent clauses.