If they're anything like typewriter ribbons of the era (and why wouldn't they be?), a really long time if you re-ink them periodically. I don't think any of my dad's fabric ribbons ever actually broke, and he typed a lot.

EDIT: Not "professional secretary" levels, but a lot. Enough that he bought a Selectric II for use at home.

Do you mean last as in how long until they dry out or last in how much you can print?

It's usually linear inches of text - which depends on how much you're printing.

Author here, I didn't! I've updated the article to reflect the actual port (and have since learned the difference between the two).