Answer
The time on the water depends on the operator and the type of service; a fast direct ferry can take about two and a half hours, and a route with a transfer through Colonia can take significantly longer. Therefore, the estimate of “about two hours” cannot be applied to every ticket between Montevideo and Buenos Aires.
Allow for check-in, immigration, and customs checks in addition to the journey time. You need to arrive at the terminal by the time specified by the carrier. During holidays and high season, queues increase, and weather conditions can affect the schedule.
When booking, check the departure and arrival ports, baggage allowance, refund conditions, and whether a bus segment is included. Do not plan an international flight too close to the ferry: leave sufficient time for checks and delays.