If your soil has food or nutrients already, then you don't necessarily need to add more fertilizer (eg, Miracle Grow's Cactus Soil). You will usually see on the bag something like "feeds for 3 months" or "9 months" etc. That tells you there is already fertilizer there.

That also means that is a guideline as to when you'll need to change out the soil. It doesn't necessarilly mean all the food has been taken up, since the roots may not yet extend to the entire pot, but use it as a guideline. So you could leave the soil longer if you wanted to.

The other guideline can be that you'll want to repot about once a year...or when the roots are getting root-bound. As you can see, there are several ways to decide when to repot, and a lot of it has to do with how fast things grow or use up nutrients, etc

Now as for specific food, some plants will benefit from other nutrients, too. (eg a plant that is about to flower, could use more than just what is in the plant food of the soil). That is why, even with plant food in the soil already, you may still want to supplement for the plant. In that case, follow the guidelines mentioned on the food package.

But let's say the soil mix is something you created on your own, without the commercial plant foods already mixed in. Then you'll want to add your own regimen of fertilizers at the recommended doses.

Post this on the Cacti and Succulents forum. There are some extremely experienced growers there that will give you advice on soils, the need (or lack of) for repotting and how/when to fertilize. Not all of these plants need the same care - many require little to no fertilization - so it helps a lot if you can supply the plant names or photos.