What "realistic" setting do you project to have to use your firearm? That alone should answer your question.
However, I'll let you know how I train. I train at these distances: 3 yds, 5 yds, 7 yds, 10 yds, 15 yds, and 25 yds. Note, these are in yards, not feet. The reason I train at these distances is because these are the distances I will be at when I do firearms qualifications.
However, what are you training for? Is it self-defense? If so, anything past 7 yards is more than likely overkill, and the way I would train is to try to get off rounds quickly and in center of mass. Target shooting so you can hit a one inch by one inch square SLOWLY is not good training for self-defense. You want to be quick, accurate (on Center of Mass), and be able to get many rounds off quickly. There are many other drills that I would recommend if interested, but you only asked about distances.
If you are trying to target shoot, then any distance would be an interesting challenge. If you have a competition you are going to join, find out from them where the targets will be and how large. This will give you the answer you are looking for.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well armed lamb contesting the vote."
~ Benjamin Franklin (maybe)