Half a day
Up to the rock by lift. The palace square, the cathedral, the Oceanographic Museum, then out to Fort Antoine. Down to the harbour and walk the quay.
That is a genuinely complete visit and it costs one museum ticket.
One day
Add the street circuit on foot — an hour, uphill, free — and either the Exotic Garden for the view or Fontvieille for the made ground.
Finish at the Japanese garden and Larvotto.
Two days, and the honest version
Two days is more than Monaco needs. The better plan is one day here and one day out — Menton, Èze or Nice are minutes away by train and are a different country in every sense.
Using Monaco as a base is expensive; visiting it from the French side is not.
Palais Princier
Musée Océanographique
Port Hercule
Larvotto