Once upon a time, I shot A LOT. I mean a whole lot. When 100 round WWB 9mm was 11 bucks a box I would go through 2 or more per week. And probably about a box of 45 per week. Ok, I know there are some here who shoot more than 300 rounds a week... but that's neither here nor there. To me I shot a good bit back in those good ole' days.
I have not had the ability to shoot that frequently in a very long time now. My family takes most of my time and money now. I wouldn't have it any other way, but I do still enjoy sport shooting. I maybe shoot 200 rounds per month, in a good year, as opposed to more than that per week. I have found that my proficiency is not what it was, however I still shoot better than 95% of other folks at a public range and was fairly competitive in the last IDPA style match I shot in.
All of this to say... If you have put a good bit of time into being proficient then you likely can get away with going to the range once a month, or six weeks. If you have not gotten to that level, you will probably need more range time to maintain proficiency. Also proficiency is subjective. I'm angry with myself if I start dropping rounds outside the 9 ring on a B27 at 25 yards (rapid fire). Some people are quite satisfied doing the same at 7 yards. Some who are better than I would be displeased to drop rounds out of the x ring at 25 yards. It just depends on what level is good enough for you.