Disregarding the legal aspect, personally I think that anybody doing less than 30 hours in a year is (a) probably dangerous, and (b) should be questioning whether they should really be doing this - or at-least, doing it the way they are.
Flying is a perishable skill, no doubt. The hardest part of flying, of course, is paying for it. From a private pilot perspective, especially post-checkride, it's hard to justify the cost of flying. Most students working toward their private have a solid goal; they can justify the cost of the rental or partnership and hourly expense because it's part of their effort to get their private pilot certificate or license.
Once the certificate is obtained, that justification is gone. A lot of students quit once they get their initial certification. The motivation has disappeared, and it's very hard to justify the expense.
Just as I always encouraged a student to fly at least once a week, more if able, I always encourage a private pilot to fly at least once a week. If that's not possible, once a month. Students are given to understand that the less the fly, the more review they will need, and the same applies to flying privately after certification. Proficiency fades, and the less someone flies, the more they should consider getting review with an instructor or another experienced pilot.
I found that I couldn't afford to fly privately, so I put myself in a position to get other people to pay me to fly. Flying once a week can get expensive. It's hard to justify, especially if someone is doing it for a hobby. This is the reason that I always encourage individuals to seek higher training. It's so much easier to justify flying and training when one is working toward a goal. Begin training toward an instrument rating, for example, or toward a commercial certificate, and there's some motivation. One doesn't need to be training every minute; one can go do hood-work or safety-pilot work with another private pilot. One can be practicing landings, chandelles, lazy-eights, approaches, stalls, steep turns, or any number of other proficiency building exercises, to justify the cost.
I've known individuals who built their own airplane or bought a homebuilt which used very little fuel, didn't cost much to maintain, and which was inexpensive to operate, in order to fly more, pay less, and stay more proficient. That's always an option.
Personally, I have a hard time staying proficient if I'm flying less than 300 hours a year. Generally I fly 600 hours or more a year, but someone else is paying me to fly. I certainly couldn't do that on my own, and I dont' think I'd try. If I go several weeks without flying, I feel the effects the next time I get in the cockpit. I have several decades of flying experience, and more than a few hours, and it doesn't immunize me from the effects of not flying. I need regular instrument flight, regular landings, regular exposure to what I do, or I feel the effects. Time away from a conventional gear airplanes will be evident, and I've been flying them since I was a teen.
One should never make any assumption regarding proficiency. People tend to overestimate themselves. The notion that it's just like riding a bicycle isn't true. One can get on a bicycle and go after a long time away from the handlebars, but there's a big difference between staying upright on a bicycle, and being a proficient pilot.
I should add that there are a lot of ways to keep your mind in the game between flights. Participating on sites such as this is one. Reading as much as you can about flying and flying topics is another. Not simply stories about people who fly, but by digesting material such as the Barry Schiff series of "The Proficient Pilot," or any number of flight manuals, is a good way to keep your mind constantly working about aerodynamics, situational awareness, navigation, performance, safety, and so forth. Studying accident reports to learn from the mistakes of others, reading the maintenance manual for the airplane you fly, studying the aircraft flight manual, etc, are all all good ways to keep your mind on flying. That's important to keeping you proficient, just as getting in the airplane is important.
Some find the use of various computer simulations or games, to be helpful. I can't speak to those as I don't play them or use them, but I've had good reports from a number of individuals who felt that it helped them.
Visualization is also important. There's a lot to be gained from sitting in a chair with one's eyes closed and visualizing one's way around the traffic pattern, or going to the airport and sitting in the airplane on the ramp and doing the same thing. I always encourage students to do that; pick a weather day when flying isn't an option, and go sit in the airplane and fly the trip in your mind, reaching for the controls, feeling the airplane, imagining the trip. You might be surprised what it can do for you, and in a world where it's extremely expensive to start the motor, it's one more way to keep yourself tuned up to the flying you'd like to be doing.
Recognize that if you can't fly much, your proficiency will slip more than you will realize. Recognizing the fact is a key to knowing in advance and acting accordingly. You're not a sharp as you think especially if you're not flying regularly. Tailor the flying you do, then to take this into account. That may mean flying with an instructor, or it may mean dedicating your one hour for the month to doing landings, rather than sight seeing. It may mean putting together a flight card before you arrive at the airport: you're scheduling exactly what will be done on the flight to get the most out of your money and your time. Three stalls, four normal landings, Five minutes of slow flight, two soft field landings, etc. Some concrete target ideas of what you're going to do will help you make the most of the time you've got.
Once a week really ought to be the minimum, but it's hard to do, and hard to justify, especially in this economic climate, and with the cost of flying today. If you can't fly once a week, shoot for once every two weeks, and do everything you can to maximize on that flying. If you can't do that, then shoot for once a month. If you can't do that, you'll be best spending your flying with an instructor to stay proficient, because you won't be flying enough to keep up your own level of proficiency. If you have very little flight experience to begin with, then you don't have much of an experience base to fall back upon, but don't forget that no matter how experienced you are, flight proficiency and skill begins to fade with disuse very quickly. Recognize it, and plan accordingly. Fly as much as you can afford, and make up the difference in enthusiasm, interest, and effort.