How often should you water cacti in winter & in summer?

what catus? Indoor or outdoors?

Whenever it rains. :)

Seriously, it shouldn't be very often, it's okay to not water for a few weeks.

All depends on where you live, where the plants are located. They actually need very little water and most people have a tendency to over water. Here is a link to find your Zone and you can go from there!

http://planthardiness.ars.usda.gov/PHZMWeb/

a friend of mine raised cacti as a hobby. he would water his when he saw on the news/weather report that Arizona got rain fall.

I water mine a few times a year. Mostly because I'm bad at remembering those types of things. Lol. It's 4 years old, and still okay.