Answer
A visa-free tourist can stay in Uruguay for up to 90 days within any 180-day period. For Russians, this is the main reference point for a short-term stay, if the trip remains a tourist trip.
These 90 days should not be treated as blanket permission for any activity in the country. Tourism differs from work, study, entrepreneurship, and the intention to reside permanently, so a change in the purpose of the trip requires different grounds and procedures.
In practice, this means that entry and exit dates should be calculated in advance, especially if several consecutive trips are planned. An error in calculating days can create problems on the next entry or when transitioning to long-term residence procedures, even if the first trip went smoothly.