I've now played 15 rated Rapid games but my percentile still shows N/A and I don't see my global rank. I see it with my Bullet and Blitz score though. I figured it would show me as active after I played enough games. Is 15 not enough? When will it assign me a global rank? Is it based off something else, like Glicko RD being low enough or something?

I think it's 20 before all your stats kick in.  I'm often wrong...

That sounds reasonable, but I think the stats kicked in at a lower number for bullet and blitz, but to be honest I'm not sure.

Well I just started playing Live after a 6 or 8 year absence, and I had only played 5 each of blitz and rapid anyway.  I do remember looking at either the blitz or rapid a few days ago when I had 19 games and noticing that my stats were incomplete.  They kicked right in at 20: or so I remember.  Of course since I don't remember which it was...

Just now confirming, yes it took 20 games for me to show up on the global rankings in Rapid.

Not true, I’ve played 20 games and I still have no global ranking, I’ve seen people with over 100 games with no global ranking before: I’m sure it’s something to do with the Glicko RD for that time control. That’s probably why the player pool for rapid is so much weaker than blitz, since a good chunk of people who play rapids don’t have a global rank and so the program isn’t solidified on whether or not your current rating is true. 

Maybe it's a premium membership thing but I found your rank pretty easily...