When I used to work in Russia, the standard was 28 days (or 4 weeks), but there were some requirements: you have to take first vacation as 2 weeks minimum, most companies expect you to submit your vacation before the year starts, and you can take vacations only in weeks. So 28 days is actually only 20 working days. As basically it counts weekends too.
In the US it was a disappointment to learn that I only had 15 working days as vacation, but after learning the system, you can always make it work way better, by combining bank holidays with vacations, you can be on vacation like 5-6 times a year, but shorter periods, which I liked way more.