Cacti do not need to be watered as often as regular plants. They can often go without water for two weeks! Did you now there are more than 2,000 varieties of cacti? And cactus spines are actually the cactus's leaves!
It is really difficult to get cacti to bloom indoors, mostly because they need intense, long periods of sunlight. Another reason is that some cacti grow slowly and won't mature for decades.
This is true of the very hairy old man cactus, which is native to Mexico. I have one of these too, but will probably never see its whitish-yellowish flower. It takes 20 years before they flower and they usually flower only when they are outdoors. But that's okay, it's messy, carefree hairstyle is flower enough for me!
I'm not sure of my cacti varieties, but I'm researching it and think that perhaps the one above is a pincushion cactus, while the one on the right is still a mystery to me. It's funny that they both have shocking pink blooms.