It really depends on the answers to the following questions:

1.) How much sun do they get?
2.) How warm are you keeping them?
3.) How big is the dish garden container?
4.) What is the consistency of the soil?
5.) Does the dish garden have a drainage hole?
6.) Are the cacti outside or inside?
7.) What time of the year is it?

Assuming you have the dish garden is in decent light (southern exposure) and the dish garden is not excessively large I would say once a week should be plenty this time of year. If your dish garden does not have a drainage hole, be very careful watering. Normally, when I water my cacti, I soak them thoroughly but you can't do this without a drainage hole.

In summer, water when the soil is dry and not before. In winter water less or not at all. Works for pretty much any cactus. Some succulents prefer to stay more moist, some like water in winter, some are dormant in summer, a few need less water than the cacti. Mostly, succulents and cacti don't mix well in a dish for very long.

Your Opuntia's will grow like weeds in comparison to the succulents, which can look like weeds, and might dominate the pot. Since the great experiment begins now with some rather expendable "plants" you will soon learn how much light they will need and how much water they will tolerate to maintain growth but not look spindley. So, I had great luck with eight different cacti in a long planter box for about five years and just repotted them this past Spring in their own six inch clay pots. They started out as 99 cent and two dollar babies and had spent many years inside getting only direct sun in Winter. My brain woke up one time and said, "Cactus live in the desert...lots of sun." So I put them out on the balcony (Southern exposure)where they got lots of direct sun but had some shade from the Iron railing and lots of water, rain water. Had to pour it out of the planter box tray after heavy rains. And the funniest thing happened, they grew. The whole factor in survival and healthy growth is drainage and temperature. I really can't be 100% safe till June 1st here in Indiana and they have to come back inside NLT October 1st. Once they are inside they will not get watered for about a month and they will never get a drenching inside like they do outside in the rain. The main factors inside are length of day and direct light and temperature. I dislike Opuntia because they are too aggressive for container gardening and they grow too big too fast (and I hate those glochids). Good luck and don't be discouraged if you lose a few along the way. Gymnocalyciums are very good bloomers and like water in the Summer. -CJ