The answer, first: scoop at least once a day, and plan a full change every two to four weeks for clumping clay or sand litter, or about once a week for non-clumping types. That is the standard advice, and on its own it is close to useless. The two numbers that actually set your interval are how much litter you pour and how much fine dust your litter throws off. Neither of them is on a calendar.
One owner runs a 25 lb bag through a large box and changes it every 35 days. Another changes every 14. Both are telling the truth. The difference is not diligence. It is fill weight and fines. A thin layer of litter saturates and powders fast. A deep layer has more working material to spare, so it holds longer, and it costs more every time you dump it.
Smell is a scooping problem. Dust is a replacement problem.
Once clay or sand has sat in a box for a few weeks, the particles that break down sift downward and stop clumping. What collects at the bottom is fines: too fine for any scoop to lift, and the reason a box can smell perfectly fine and still be finished. Owners who track this report the same trigger. The cue to dump the box is that the fines have built up, not that the box reeks. Wait for the smell and you have already kept the box past its useful life. Change purely on the calendar and you have probably thrown out litter that still had two weeks left in it.
So weigh one fill. Do it once. After that, "how often" turns into a consumption rate you can measure. A single cat on a large box runs somewhere between 10 and 15 kg a month once top-ups are counted. Another household pushed 5 to 6 bags of 4.5 kg through a single box in the same span. Three cats on four boxes is a different sport entirely: 90 kg to fill, plus more than 20 kg of top-ups before the month was out.
The same 25 lb fill that lasts about a month in a dry winter falls to about three weeks in summer. The reason owners give is not odour. It is bacteria. In properly humid climates one fill may not make a month at all, and a top-up partway through is what carries it. Top up weekly, with the same litter, never a different type mid-cycle. Fresh litter over old also keeps the surface at the depth the cat expects, so the box stays usable while the fines pile up underneath.
Tofu litter is the one that punishes a monthly habit. It powders as it is used and, in heat, turns sour well before a clay litter would. A week is not unusual in a warm room.
Pure tofu takes urine well but does not grip faeces, which is why so many owners keep a bentonite fraction in the mix rather than going all tofu.
A litter that worked for a year can turn on you after a reformulation. Same bag, same label, more dust. Routine is not a guarantee.
Cats decide before you do. A cat that holds it, or goes elsewhere, is telling you the box is stale while it still looks acceptable. One owner's cat would urinate on the bed rather than use a box that was a few days overdue.
And dust reaches the floor before you notice it in the air. Pale, dry paw prints on the floor after a visit mean the litter is throwing fines. That is your cue.
Weigh one fill. Note the depth. Scoop daily, top up weekly with the same litter, then let dust, not the calendar, tell you when the box is finished, inside a window of about three weeks in hot, humid weather and about a month in cold, dry weather. If you are dumping litter that still clumps cleanly, you are paying twice for the same bag.