I think you start to get in to trouble the moment that holiday stops being seen as remuneration. When it's clear that some people have negotiated different holiday allowances, and have different holiday needs, and that's been factored in to their take home, then everyone knows that everyone has earned their holiday.

As for minimums, at any European company with a vaguely competent HR function, unused holiday will get flagged as a risk in terms both of resource planning, but also from a staff wellbeing perspective.