Admission is a modest gate price set in pesos, in the same band as the island's other small municipal attractions — a few hundred pesos or less per adult, with reductions for children. It is charged at the door, and the rate moves. Confirm before you go rather than trusting any published figure, including this one.
Opening is daily on broadly standard museum hours, opening mid-morning and closing in the late afternoon. It is not a late-opening venue, which matters if you are counting on it as a post-beach option. Hours have varied over the years and can shift for public holidays, so check on the day.
Allow forty-five minutes to see it properly and ninety if you read everything. It is the most reliable rainy-hour option in San Miguel: fully indoors apart from the courtyard and the terrace, and a short walk from the ferry pier and the main square.
The rooftop terrace has generally operated as a café. Whether it is serving on any given day has varied, so treat food there as a bonus rather than a plan.