I usually wind up spending 3 hours or so in the gardens, the house and the gift shop, lol.
Last time I went I caught the 10:11 direct train to Giverny. The train was late out of the station due to some kind of signaling issue so got to Giverny about 1200. The shuttles were out of sync and wound up in Giverny about 1220-30. Ate lunch at the nearby Nympheas Restaurant and entered the garden with a timed entry for 1330. Left about 415 to catch the 1651 train from Vernon back to Paris. It was late October so was dark by then anyway.
IN Giverny I suggest you also walk down the Main Street to the church where Monet and family are buried. There is a touching memorial to a Lancaster bomber crew that was shot down on the night of June 7/8 with all crew killed.
https://ibccdigitalarchive.lincoln.ac.uk/collections/document/18681