OK, it varies. It varies only 3-4 years according to your experience. If you put the skater's practice time in hours it might vary less. Obviously a kid who is homeschooled and skates 3 hours a day and does off ice at least an hour a day is going to progress more quickly than a kid who goes to school and practices 2 hours per day.
Well, what's impossible to know is IF they will learn certain skills. But it's not impossible to know that IF they learn them, there's a range of when they will learn them.
I'm going to answer with an example of various fields, and then try to forget all about this topic (while I agree it would be a pity to see it delete):
I'd probably laugh out loud if you, as a parent of a
very talented 7yr old kid in piano who studies a lot (meaning they can play a Mozart Sonatina, or maybe a Song Without Words by Mendelssohn, and this would be frankly amazing and pushing it!!) go to me as their teacher and ask when can your kid learn the 3rd Piano Concerto of Rachmaninoff.
If I'm teaching maths and a really good talented 7yr old kid with amazing abilities (maybe he can do some crazy stuff like multiplying double digits from his head) asks me when he can prove Goldberg's Theorem or something similar (apart from being very impressed) I'll have, still, no answer.
If a really cultured and interested in writing kid (or probably his parents) asks me when he can write something comparable to James Joyce's Ulises, again, same non-answer. Or write something worth a National Prize —may be a better comparison.
But anyway, with that kind of ages is really difficult to answer, because you need to take multiple things into account, some that don't even have to do anything to do with the kid itself. Some that do with the difficulty that goes with maintaining a good attitude and interest for
years. Specially true for very active kids.
They may learn how to make a sonnet by the time they are 8,...so what?
Don't take this as discouragement to yourself or your kid. It's just that there are certain kind of activities that can't be measured accurately (more than "they'll learn it at some point, if any, before their bodies get too old). This was all the point of the topic, before it went crazy.
As for the risks and stuff, I'm no expert, I'm not a parent, and even if I was, I think it's also highly personal. It's not only your kid but also your dedication that also makes a difference.
I do seem to find that kids having a lot of activities are more stressed that those than focus in two or three (I also talk about my own experience here), but two sports are a good choice, specially if they're somehow related.
Best of luck to you and your kid. Happy skating!