I never actually know how long it takes before it appears graphically. 
I'm not sure if there is a period for any major bugs to be ironed out before the masses get the offer for the update... or if the repository lists are not fully up-to-date on your system.
But if you're craving your hands for it, this is the command:
sudo do-release-upgrade
I always recommend a backup of the whole system, just in case you need to return to Vivid. Clonezilla Live is a good choice.
Also, a special note for this release, if you have an AMD graphics card with their proprietary driver, you must use the open source drivers for now, due to a bug in the kernel.