They are unlikely to be variegated at all, alas. So far as I have been able to find, variegation happens from time to time in seedlings but is profoundly unlikely to happen when variegated plants produce seed. None of my seedlings from variegated plants produced any variegated seedlings and I planted nearly 100 of them.

That said, variegation in adeniums may be caused by them being chimeric rather than it being genetic. If that's the case, it won't be passed through offspring seed and can only be reproduced through cuttings, grafting, etc. But more study is required.

Alternatively if it can be passed on to offspring and isn't a chimeric trait, then there are likely white masker genes present. These genes have been observed in both plants and animals. Unless you get lucky to both have the variegated genes present, and an absence of the masker gene (which is dominant in most plants and very difficult to strip out entirely) then again the plants can't produce variegated offspring. So in that case it's still going to be rare and hit and miss. In some other kinds of plants, like variegated African Violets, it is further complicated by being a sex linked trait. So it also requires the 'mother' plant to be variegated to even have a chance of producing variegated offspring, in addition to the masker gene complication.

Plant genetics is still a fascinating field. All of the above said, variegation has nonetheless been seen to show up later as adenium seedlings mature but it seems to usually show up fairly early. Others I'm sure can input more data on this topic. There are also several distinctly different variegated patterns observed in adeniums and these usually have differing causes.