The NGVI is a great place to spend anything from an hour to a day, depending on your degree of interest - I close to a day for me, and never a dull moment. Very good mixed bag of classical and contemporary art in the collection, and some of the travelling exhibitions are just fantastic. Built in the 1960's and then re-imagined some time after that, the building is a very interesting space in it's own right and the cafes are of good quality when you need refreshments. Also check out the NGV Australia, over in Federation Square as well - it is for Australian Art only and also definitely worth a look if you have even only the remotest interest in art as it is as varied and interesting, in it's own way, as the NGVI.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.