My (limited) experienced is that for "regular" employees, one week per year is common... but for "executives" often six months to one year is common, despite their tenure and despite how much they personally were responsible for fouling things up. :-)
Heck, some scummy employers tend to hold onto an employee's last paycheck if the employee leaves under bad terms; they know that most employees are gonig to be busy looking for another job and likely won't have the time and/or money to pursue getting it in court.
Right, since it's "their" time. AIUI this is the reason many companies cap vacation time at, e.g., 4 weeks -- they don't want employees building up a little $100k "vacation cash" fund that tends to look bad on the books.
---Joel