Auto fill blog excerpt with first X number of words from the blog

Hi all,

I'm building a site for someone who will post a lot of blogs and am trying to work out if there's a shortcut I can build in so that the excerpt can be auto filled with, say, the first 100 words of the blog (saving them the step of the process where they'd copy it over while writing a blog).

I realise it'll cut sentences in half, and won't be super pretty... but...

I'm building it now so on 7.1, and looking primarily at the summary block (but if it's do-able on the primary blog page, that'd be interesting, too)

I'd be super grateful if anyone knew if it was even possible or just out of the question.

Thanks!