Every foreigner who comes to Russia should have his/her Russian visa registered within 7 working days upon arrival, excluding holidays and weekends. Before it is 7 days after your arrival, your tickets (train, bus, plane tickets) to the place you're in (e.g. Moscow) is the document, which can confirm how long you've been staying there.

Please, note, that before 25 March 2011 you had to be registered within 72 hours upon arrival (excluding weekends and holidays), so it might be that not all the authorities are aware about this new rule yet. In this case, please, refer to the Federal Law #42-FZ "On amendments to the federal migration law for foreign citizens and aliens" from the 20th of March 2011. This law came into force on the 25th of March and is available online on Rossiyskaya Gazeta website (the official Russian law publication).