Quality is what matters. At one extreme, I've prepped side mount divers who were able to start and then rock cave training in side mount after just a half dozen side mount dives prior to class. Another 50 full cave dives and they'd be ready to consider switching to a side mount CCR. However, they were exceptions in that they were already thinking divers, who were very solid OW divers, dedicated to good buoyancy and trim. At the other extreme I've spent 3-4 months of weekends working with other previously (and very poorly) trained side mount divers before they were anywhere near ready for cave training, let alone side mount CCR.
Beyond that it really depends. At a minimum, side mount configuration needs to be second nature so the student has his or her full bandwidth available to learn the side mount CCR skills. And, in practical terms, you need to be side mount proficient as if you ever have to completely bail out of he loop, you will be exiting in side mount open circuit while still having to manage the gas volume of your CCR.
It also depends on the unit as some are a bit more task loading than others. For example, experienced side mount divers seem to trim out in a KISS Sidewinder after about 5 minutes in the water, since it is a very symmetrical configuration. On the other hand the hose routing is pretty cluttered and assembly is fairly complicated, so it has its own set of challenges. In contrast the KISS Sidekick is quick and easy to assemble, has clean hose routing and has minimal impact on the tank on the other side. But lateral trim requires a lot more work and it is very sensitive once it is established. Consequently, it takes longer in the water to get comfortable with it. However, if you are planning on true bottle off side mount diving, the Sidekick is probably a better choice than the Sidewinder.
However, the bottom line is that it won't matter what you or I think - what will drive the process is whether you can convince a side mount CCR instructor that you are sufficiently proficient in a side mount configuration to be able to manage the task loading that will come with learning to dive a side mount CCR.
