The first year it sold out super fast. Last year it was open longer. I hate to say exactly but I know it was open longer than several days. I just don't remember. It is safe to say that it won't close in the first hour like other races. I will register for it as soon as I have my other races locked in first.
Be sure to have your cruise confirmation number handy before you try to register. If you are not online at the stroke of noon, don't panic.
I assume by your question that you have not done it before. I can tell you that it is a lot of fun. We gather in the theater very early in our assigned corral areas (A, B, and C). They walk us off the ship together in the dark! It is a long walk to the start of the race but it is soooo cool. Actually, it is very warm, but you know what I mean. The course is very interesting and there are characters to have pictures with and funny entertainment. There is not exactly a time limit but you can't just stroll if you want good pictures at the finish line with Mickey and Minnie because they leave early. There seem to be a lot of walkers and people just taking it all in. If you are looking for a super competitive race, this is not it! Even Jeff Galloway was taking it pretty easy with a slower group of run/walkers. The medals are very cool, too!
After we finish, the regular 5k for the rest of the ship starts. Those people have it rough because it is HOT by then. They don't get the same bling that we do, but they don't have to pay either! You will have to go back to the ship because the island isn't open yet. That was very nice for us because we wanted to go back and change to beach clothes and have a nice breakfast before heading back. Lots of walking back and forth but completely worth it.
Something else that might interest you is that one of the gift shops sells Challenge merch. The first year the line was really long and people (me) waited over an hour for the shop to open. They cannot open until the ship sets sail so we missed all the sailaway festivities that year while we were in line. That wasn't really so awful because it was pouring rain! We discovered later that we had waited in vain because they had plenty of merchandise and there was no issue with selling out early. Last year we waited until later to shop and had no trouble getting the shirts we wanted.
Last thing to be aware of is that you will have to pick up your packet at the Expo. You will get your bib and shirt and other instructions. It could be easy to forget about that and walk out without your stuff.
You will love it! I would be happy to answer any other questions if I can.
Lynda