If depriving cactus of light is the worst thing you can do to it, not watering it is the second worst. For the uninitiated, I think this is the way the reasoning goes: cactus live in the desert; deserts are dry; therefore this cactus can stay dry and doesn’t need water. Nothing could be further from the truth. Cactus can go for extended periods without water, certainly, but they do need to be watered. A cactus planted in the ground is able to find whatever moisture might be lurking in the dirt and can even absorb some moisture out of the air. Cactus planted in a pot and perhaps put in the house obviously can’t do that. While cactus can survive without water, at least for a while, they will thrive with some water.
So here is what I do. In the summer I water at least once a week, more if it has been oppressively hot. I water enough that the water drains through and out the bottom of the pot. You’ll want to do that so that all the roots from top to bottom get some water and not just a few of the roots in the top of the pot. The trick is to make sure the drainage is good and the pot doesn’t sit in the run-off water that might collect in a saucer. That’s the thing with cactus: they like some water but don’t want to stand in it. Water well and then let them dry out.
The next problem is, of course, how do you know when they have dried out? Over time it is just something you get a feel for, but one thing you can do is go to the grocery store and buy a package of those bamboo skewers and use one as a guide. Poke it all the way down into the dirt in the pot and see if it comes back damp and with bits of dirt sticking to it. If it does, you might let it dry out some more before the next watering. If you live in a humid area that gets lots of rain, the rain will take care of a lot of the watering for you. Cactus and succulents do love a good rain shower. The trick, again, is to not let them stand in the water afterward and give them plenty of time to dry out.
In the winter you will want to cut back on the water, but they will still need some.
Now, all of that having been said, if here is ever any question, you will want to err on the side of dry. Just don’t expect them to do well without any water at all! Occasionally you may rot one; it happens. I have lost a few over the years, but that’s how you get the hang of it.