I do weekly planning and have a family of 4, including two teenaged boys -- I can't imagine finding the refrigerator space for more than a week's worth of vegetables.
Our meals tend to be vegetable-heavy. We eat meat, but try for several vegetarian dinners per week.
Breakfasts: We have an assortment of stuff around, so this is mostly make-your-own, but if asked nicely I will make baked oatmeal (there are tons of recipes online) with tons of fruit and leave it in the oven programmed to start baking at 5:45 or so, so it's ready to take out at 6:30. (This only works in the winter -- our kitchen is quite cool in the evenings, so I don't worry about leaving this overnight in a cold oven.)
Lunches: I solicit suggestions Fridays/Saturday mornings before making a list. Some negotiations happen, but at this point we have agreed on some tried-and-true standards. Usually my husband has sandwiches, and the kids have burritos/quesidillas/rice-and-beans with salsa and chips/leftovers. MY lunches are often varieties of chopped salads (this week it's fennel and oranges with lemon juice and pepper) and toast with something. I make sure we don't run out of un-mustachian very good bread.
Dinners: I primarily shop at a local store with decent prices, and supplement with a stop at Trader Joe's (for cheese, and various odds and ends). I look at the weekly flier for my store, and base meals on whatever meats and vegetables are on sale. At this point, I look through whatever cookbooks/notebooks/websites feel inspiring to me that day. So this week we're having:
mixed grill, sautéed snap peas, salad, cheesecake (it was my BIL's birthday)
chili (with part of the leftover grilled meat and beans), tortillas, salad
pork and veg stew (very light on the pork, with the remainder of the leftover meat)
turkey enchiladas, cabbage-and-carrot slaw (I shredded the turkey and put it in the freezer last month)
polenta with spicy cauliflower and tomato sauce
linguine with artichoke sauce (and green beans)
(it appears we're missing one here, but I'm too lazy to go see)
Once the menu has been shopped for (there are usually ample opportunities to substitute things that are discovered to be on sale) the dinners are written on a whiteboard on the refrigerator. That way everyone (including me) can see what the available dinner options are, and it's easy to think "hmm, today we're all going to come home late, so it should be something in a crock-pot" or whatever.
We tend toward lots of room-temperature salad-y meals during fall and spring, AKA soccer season, along with other things that can be made well ahead. I print out recipes that look interesting, and they often get one try before they're either tossed or stuck into my notebook.
We eat an enormous variety of things: nothing that we're eating this week was something we ate last week, although I believe we had turkey enchiladas the week before. Right now, we tend to have one or two mexican-ish things per week, one or two vegetarian things per week, possibly one grilled meat thing per week, and chinese/indian/thai - inspired stuff once per week. We've also created an entire family of "nicoise-inspired" salads, which are potatoes and vegetable-based with grilled meat, possibly tomatoes, something pickled, and something appropriate in the dressing (so a green goddess-type dressing with fish, and a mustardy vinaigrette if we're having potato, cabbage, and sausage as the base).
If I were trying to plan for a month, I think I'd shoot for two weeks of meals twice and aim for cooking double-batches and freezing some of the things -- anything stew-y or sauc-y will probably freeze OK, and then half the work is done. You also have the chance to plan inter-laced meals to be sure to use up everything (the classical example of this is roast chicken the first night, chicken salad sandwiches the second night, and chicken soup the third night, but shredded chicken and chicken stock both freeze well -- as do chicken bones if you're not in the mood to make stock right then and there).