It's not about the length more about the content. If it's random rambling I usually quit by the first 5 lines or so.

If someone wrote a novel...an interesting novel...I'd sit and read the whole thing.
Other people can post 10 words and I can't make it past the first 5 LOL
More than two reasonable paragraphs is pushing it though.

Longer than 5 paragraphs maybe. The style of writing or the topic really determine how long it will keep my interest.

Depends what it's about, I got bored of biscuits very quickly

Depends on the content

Thats actually my limit