Actually I think it is sufficient ... I only go out about that frequency .. I shoot just as well as I did during the extensive training period .... once you have a gun down to its mechanics, quirks, sighting, etc..you don't need to go out that often.
One you have proficiency once then every 6 mo. is fine for some people. I doubt people go out and train like special forces ... with most people. There are simply no facilities to do that for the vast majority of people (to train like they do). They are professional shooters basically. I would think that civilian professional shooters shoot their guns as often as special forces folks do.
I think you are missing my point - proficiency is about more than accuracy. If you consider being proficient with your firearm to mean being able to put x rounds inside of a y" circle then fine, but to me that is only the barest of beginnings.
Again, not saying you have to shoot or train as much as a professional shooter or special forces but a handful of rounds every 6 months does not make you proficient in my mind. To be proficient you must train and practice more than just target shooting regularly - clearing malfunctions, reload drills, move-and-shoot, shooting from cover, weak-hand drills, target identification, drawing from carry position, drawing from concealment (if you carry that way) wearing the same clothing you typically wear, using the same weapons you typically carry, using the holster or method of carry you typically use, and more - and doing all of that with good sight picture, trigger control, follow-through, etc.
It is a fact of life that if you don't maintain skills they degrade, so you must regularly train and practice to keep the same level of proficiency as you currently possess. A box of ammo at a stationary target every 6 months doesn't cut it in my opinion, and honestly I think you do yourself a disservice if that is the extent of your proficiency.