An old-time greenhouse grower taught me their way of measuring moisture levels in granulated soilless media (including coir, soil, and peat-based soilless mix). It took me a little while to really trust it, but I now prefer it to anything except the very top end moisture detection equipment—and I would still use it to see if those were working properly.
Try this experiment. Find a plant with a granulated media (like coir, peat-based mix, or potting soil). Take a handful of media and squeeze it in your hand very hard. Observe what happens, then open your hand and look at the media. Compare your results with the list below.
1. Very dry. You cannot squeeze anything at all out, and the sample in your hand breaks into many pieces (or does not stick together at all) when you open your hand. I don’t know of any plants that like their soil this dry, but I have little experience with deep desert cacti which might like it.
2. Dry. You cannot squeeze any water out (or possibly can work one drop out if you’re grip is very strong). When you open your hand, the media breaks up into several pieces and some parts might crumble apart. (For 90% of plants, you should definitely water it now, but add the water slowly to avoid tip burn).
3. Moist. It takes a few seconds to get water out, but 4 to 8 drops are worked out of the soil with a reasonable amount of force. Squeezing very hard might get a couple more. When you open your hand the media usually breaks into one or two pieces. This is the perfect moisture for almost all dry land (ie. not swamp) plants. If you can keep the plants at this level all the time, that’s great.
4. Wet. It is not hard to get water out of the media. You squeeze and a steady stream of drops comes out for several seconds. When you open your hand, the media stays in one big chunk. This is too wet for most plants and if it stays this way for very long it will likely cause root damage. Sometimes right after you water, you may have this condition so I’d recommend checking 20 min or more after you water to see if the moisture level is still this wet. If you have good media, it will drain to level 3 wetness. Some plants from very wet environments might like this, but it’s rare. If any research on your plant says something like “likes well-drained soil” (or something to that effect) your plant will not like being this wet for very long.
5. Soaked. When you pick up the media, water may already start running out. When you squeeze it, you get a stream of water. When you open your hand, some media will not hold form because of the moisture and some will stay tightly together. Either way, it’s too wet.
Almost all plants would like to stay in the 3 range all the time. They can handle short periods in 4, and short periods in 2 but really, they thrive best in range 3. Staying in 3 all the time requires a very diligent indoor gardener or a good hydroponic system and also good media. This is one of the reasons that I don’t recommend soil (even potting soil) for indoors.
It is harder to keep plants in potting soil at level 3 moisture because soil rarely has enough porosity (air space), so it more easily holds the plant at level 4. I suspect that the (mistaken) common wisdom to let your plant dry out between waterings developed because people used to always grow their plants in soil media in containers (and would have had a hard time keeping the plant at the “3” level).
Well-informed indoor gardeners try to find media that holds water well, but also holds air well. Soil rarely does this. Consider soilless mix (as coarse as possible), coir (coconut fiber), peat moss, or stone wool. It also helps to understand how plants drink.