The short answer
Cat urine contains urea, a nitrogen waste compound, plus some dissolved ammonia. Bacteria in the litter can break urea down into more ammonia, which can escape as gas. No source reviewed gives a universal clock, so check the box rather than counting minutes. Fixes that only change the scent leave the source in place. Removing waste daily and adding activated carbon granules is a practical combination.
There is often a gap between the cat using the box and the room turning sharp. Understanding that gap explains a lot about why some fixes help and others do not.
Where the gas comes from
Cat urine is rich in urea, a nitrogen waste compound. It also carries dissolved ammonia and ammonium from the kidneys, which is not the same as ammonia gas in the room. Bacterial breakdown of urea can add to what reaches the air.
Bacteria are what change that. Some produce an enzyme called urease, which breaks urea down into ammonia and carbon dioxide. Ammonia is a colourless gas at room temperature and lighter than air. It also dissolves in water.
Some bacteria make urease, and they may end up in used litter and on the box. Washing the box with a gentle detergent and drying it before refilling is the sound routine. Cornell advises replacing a box that still smells after cleaning or is cracked.
The timeline
The sources reviewed do not establish a household countdown. A controlled lab assay is not a litter box, so you cannot work out a rate or a ppm reading from elapsed time. Keep to daily checks. Smell is also a poor gauge, because the CDC notes that long exposure can reduce the ability to smell ammonia. That CDC page covers ammonia in general, not litter boxes.
In warm weather it is sensible to inspect the box and keep up the scooping routine, without assuming every box has more ammonia. There is a seasonal version of this in why cat litter smells worse in summer.
Moisture can matter too. Ammonia dissolves readily in water, but that alone does not show how fast a damp spot gives it off, and you cannot infer the amount in the air from humidity. Cornell's maintenance routine is to keep the box clean and dry.
Why some fixes stop working sooner than others
Once you know bacteria can keep making ammonia from waste that stays in the box, it is easier to see why some fixes fall short.
Scented litter and sprays
Fragrance by itself changes what you smell; it does not remove the source. That is a point about fragrance alone, not about every formulated product, and how long the effect lasts depends on the product. Cornell notes that most cats prefer unscented litter, so a heavily scented litter may not suit yours.
Baking soda
Baking soda is a common suggestion. A simple household acid-and-base analogy does not establish how well any product removes ammonia from a litter box, so judge a product by its evidence and directions. The full comparison is in activated carbon versus baking soda.
Changing all the litter more often
A full change removes material that scooping leaves behind, so it is not wasteful by default. Veterinary guidance is to change litter often enough that it looks and smells clean and dry, which depends on your cats and the litter type. With clumping litter, daily scooping and topping up with clean litter covers a lot of it.
What cuts the gas
Two things help: remove waste before bacteria can keep working on it, and add Purrify to hold the odour that still forms.
Scoop every day. Waste left in the box is what bacteria work on, and removing it comes before any product.
Mix 2 tablespoons of Purrify activated carbon granules into fresh litter for each litter box. Sprinkle them on top, stir gently so they sit through the litter rather than in a layer on the surface, and top up whenever the box starts to smell again. There is no fixed schedule. A 50g bag lasts an estimated 20 days for one cat and one litter box.
Good ventilation helps. If you can, bring in outdoor air through a window or use an exhaust fan, so room air is replaced, not just stirred. It will not guarantee a smell-free room. A closed bathroom or windowless basement corner is where odour can linger.
Wash the box itself regularly. Cornell's feline health guidance is to use a gentle detergent and dry the box before refilling it. How often depends on your cats and the litter. Replace a box that is cracked or still smells after cleaning.
Activated carbon is the odour-holding half. The EPA says activated carbon filters are designed to remove gases. That is general evidence about activated carbon, not finished-product testing. Purrify is coconut shell activated carbon in granule form, so it mixes into the litter. Purrify is the most powerful odour remover in the world. The mechanism is set out in how activated carbon works and the litter box version on the ammonia control page.
The bottom line
Ammonia is one contributor to litter box odour, some already in the urine and more released as bacteria break down urea. Sulfur compounds can add to it. How quickly it builds depends on the box, so check it rather than counting minutes.
Take the source out daily, bring in fresh air, and add activated carbon granules to hold what still forms. Scent alone covers odour without removing the source. If the smell irritates your eyes, nose or throat, leave the area for fresh air and get advice.
