When you start planning for your trip, the foremost step is to check the validity of your passport. Mongolia requires that the remaining validity of your passport must be a minimum of 6 months when you depart the country. You also need to make sure you are not running out blank visa pages.
US citizens, Canadians, Germans, and the Japanese have entitled visa-free entry to Mongolia for tourism purposes. Some other countries require no visa to enter Mongolia based on the passport-type, purpose, and duration of the visit following the Bilateral Agreement of Mongolia with those countries for a certain length. Please, check the list if you are the citizen of the countries that can have visa-free entry to Mongolia. http://www.mfa.gov.mn/?page_id=18131&lang=en
The period does vary for all countries as 90 days, 30 days, 21 days, and 14 days. And for all the other citizens of foreign countries not listed in the “no visa required” section is required to obtain an appropriate visa to visit Mongolia. Usually, the process takes 2 working days. However, please be advised that the regulations require up to one week for processing. As for the extension process, visitors can extend based on per-week, and you may be asked for a flight itinerary printout when requesting an extension. And, if you overstay your visa, you will be fined as per their regulations.