Email Spinning

Hi Guys

Just quick question.

When sending out e-mails alot of the time E-mail spinning is required to change up the content of the e-mail so it bypasses spam filters if that content has been flagged before.

My question is, by how much does the content have to be spun and how can you tell if the new content has been spun enough not to be flagged along with the previous batch of e-mails sent?

Thanks!