I don't think there's a good way to implement it, unless it's actually unlimited, i.e. you can stay at home for two years while getting paid and it's fine.

If that's not fine, then it's not actually unlimited, and you have to guess where the limit is - so you can't really be guaranteed a certain number of days off.

Though I guess an org could still set a minimum and then call it unlimited, though in practice I expect that that minimum is basically also the maximum.