Continuing training is vital to continued skill. If you want to carry a gun for self protection, you have to invest in good training and time spent keeping that training up to date.
According to Wikipedia Massad Ayoob is an internationally known firearms and self-defense instructor. He has taught police techniques and civilian self-defense to both law enforcement officers and private citizens since 1974.
He was the director of the Lethal Force Institute (LFI) in Concord, New Hampshire from 1981 to 2009. Ayoob now directs the Massad Ayoob Group (MAG).
Additionally, Mr. Ayoob writes some of the best firearm books on the subject of the legal aftermath of a self defense shooting.
Furthermore, I widely quote his work in my classes, and recommend his books courses and videos to my students.
With many things I learn a skill, try it out, have fun with it, and then put it on the shelf until I need it. The concept works well for basic skills that are rarely used and have low consequence for failure. In contrast, this does not work when failure is not an option. With skills like the defensive use of a firearm continuing training is vital. Finally, shooting is a perishable skill, shooing for self defense is complicated and has extreme penalties for failure.