When is "tweet your answer" button available?

Question

I was trying to reproduce and document the bug I reported in my question

• Tweet button from the same answer always gives distinct link

I opened all my answers in EL&U one after another and this button did not appear (again) in any even from threads published by @StackEnglish, for example,

• What does "Alien" actually mean?

When does this functionality of tweeting button appear in an answer?
For example, I it did appear before while I tweeted:

• My answer to Stack Exchange English Language and Usage Q: What is a term for a system with rules without any exceptions?

And even in that answer this button/link of tweeting does not reappear.
Though it would have been convenient to preserve it for multiple tweeting or getting a link for copying to other boards.

Answer 1

It is available

• a certain random percentage of the time, scaled to your reputation
• for about an hour
• if your post is > 400 chars
• on your posts only (eg it is only visible to you)