Here's how to think about it:
* Purpose: Is the quotation meant to be a short, impactful snippet? Or is it meant to be a more detailed excerpt of the original source?
* Context: How long is the surrounding text? A quote that's brief within a long article might seem verbose in a short tweet.
General guidelines:
* Short and sweet: 1-5 words might be considered very brief.
* A short sentence or two: 5-15 words is likely to be seen as brief.
* A longer excerpt: More than 15 words might be considered a longer quotation.
Ultimately, it's about what feels appropriate for your specific needs.