When creating a dynamic rss feed do I need to include a full history?

When I create entries for my开发者_开发问答 RSS feed should I include a full history or just partial?

How many should I go back?

Do rss readers remember entries in the past (before cutoff) if I make it a partial feed?

Thank, Kevin

Obviously it's impractical to keep a full history in your RSS feed if you have a frequently updated site. Sites like the BBC would end up with RSS feeds several MB long if that were the case.

I generally limit my feed to the last 30-40 posts—people use feeds to stay up to date with the latest news/announcements from a site, so they really don't need to see feed items from 2 months ago. But sites like BBC news seem to maintain feeds around 80 items long.

And I don't think most readers remember past entries. It's sort of assumed that old entries will eventually fall off the feed as new items are added. RSS is for syndication not archiving.