The character creation restriction is fairly simplistic in that it just looks at how many players are currently logged in on the world you're attempting to join, and if that's above some threshold, then you can't create a new character there. Since that's continually changing as people log in and out, the restriction can start or stop at any time, but picking a low-traffic time (like early morning weekdays) gives you a better chance than trying during peak hours.

(I've heard that Balmung is so overcrowded that this doesn't work there anymore. It's basically always full at any hour of the day or night. But I think any other server should still open up periodically. So a bit of persistence should be able to get you a character on Cerberus.)