They quote 50 miles to cover themselves, but runflats have been driven 300 miles. Obviously that destroys them and they need to be replaced rather than repaired, but you are not strictly limited to 50 miles. The main problem is France on a Sunday when most tyre shops are shut, but since most BMWs are now on runflats, as long as you stick to sensible standard sizes, you should be able to get replacements.