There is no fixed number of days. Push a finger into the pot and water only when the top inch (2.5 cm) of soil feels dry, and ignore the calendar while you do that. One check is the core of watering indoor cactus. In summer your gap might be a week. In winter the same plant can sit two to three weeks between drinks. PlantTalk Colorado, from CSU Extension, sets the top-inch test as your trigger and tells you to move the gap with the season.
My first cactus grew a pinched, narrow waist with a fat top above it, and the whole plant leaned out of a terracotta pot with a chipped rim. I tried a splash from a mug every second day through that summer. None of the water ever got past the top layer of grit. A run of repeated shallow sprinklings often ends in distorted growth. Mine never straightened out.
A fixed cactus watering schedule fails for one reason. Your plant will not tell you when the schedule is wrong. Cacti do not wilt under drought stress, so you get almost no warning before the damage is done. A leafy houseplant flops over and you fix it that afternoon. A cactus keeps its shape while the roots rot in wet soil underneath. Light, frequent sprinkles make that worse, because they wet the surface and leave the roots below dry.
Judging dryness from the surface is where you slip. A gritty mix goes pale and dusty on top within a day, while the soil an inch down stays damp. Push your finger in to the first knuckle instead of trusting your eyes. If you feel any cool moisture down there, leave the pot alone and check again in a few days.
Your gap between waterings is not a number you set once and keep. A cactus in a bright window in July, in a small clay pot, dries out far faster than the same plant in a cool January room. Warmth, light and pot size all move your number. So you check the soil rather than count days, and you let the interval shift on its own.
Two habits cover watering indoor cactus for good. Check before you pour, then pour thoroughly. Carry the pot to your sink and water until you see the flow run freely from the drainage holes, then let the pot drain right through before you put it back on the shelf. Empty the saucer once the dripping stops, because your pot must never stand in water. Waterlogged soil rots the roots fast, and overwatering cactus plants is the most common problem you hit indoors. Your mix matters as much as your habit. Equal volumes of peat-based potting soil and very coarse sand or grit let a full soak drain away instead of sitting around the roots.
Tip your saucer out after every soak. A pot left standing in its own runoff rots from the bottom up, and a cactus gives you few cues before that damage is done.
Season sets your interval, and that is the only moving part of watering indoor cactus. Through spring and summer, soak the pot each time your top inch dries out. In winter your plant is dormant, so let the mix get very dry between waterings and give just enough to stop the shriveling. A holiday cactus wants the opposite hand from you. Keep the soil evenly moist while your plant is budding and flowering, because a holiday cactus should never sit dry.
Read the full article: Cacti Are Not Hard to Kill, Just Hard to Read
