How much swim training is really needed to complete a 2.4-mile swim for Ironman?

This totally depends on how much swimming background you have. The swim in an Ironman is the shortest of the three disciplines but can have a huge impact on the rest of the race in terms of the physical and mental energy cost, so I think it is sometimes underestimated in that regard, especially by novice swimmers. I would say for most people who can swim reasonably competently, 2-3 times per week would be plenty if just looking to ‘complete’ the event. That said, I would still make sure you practice in open water and also get used to the long, continuous nature of the Ironman swim by getting in some 3k+ straight swims in training prior to the event. If you are a complete novice, you may need to invest a little more in the swim earlier on (10-12 months out from the event), by getting some decent coaching. This will give you confidence in that swim element and prevent you from running into difficulties on race day.