How often should I actually water houseplants?

On a schedule is the wrong approach, and it is the main reason houseplants die. Water use depends on light, temperature, humidity, pot size, soil type, season and the plant itself, and all of those change. A fixed weekly routine is either too much in winter or too little in summer.

Check the soil instead. Push a finger a few centimetres in: if it is damp, wait. For most common houseplants the correct time is when the top two to three centimetres have dried. Succulents and cacti want to dry out almost completely; ferns and other moisture-lovers prefer consistently damp but not wet.

Weight is a useful proxy once you learn it. A pot that has just been watered is noticeably heavier than one that has dried, and lifting it becomes a fast reliable check.

How you water matters as much as when. Water thoroughly until it runs from the drainage hole, which wets the whole root ball and flushes accumulated mineral salts. Small frequent sips wet only the surface, encouraging shallow roots and leaving the lower root mass dry. Empty the saucer afterwards, since standing water is the mechanism by which overwatering kills.

Season changes everything. Most houseplants grow little in winter, with less light and lower temperatures, so water use can fall by half or more. Continuing summer watering into winter is a classic cause of loss.

Two practical points: cheap moisture meters are unreliable and a finger is better, and drainage matters more than watering technique. A pot without a drainage hole makes correct watering nearly impossible regardless of care.