When does Tumblr Release a URL

When Does Tumblr Release a URL

I deleted one of my side blogs so I could create a new primary blog with the URL. When I tried to create the new account with the old URL, it says that the URL is already taken. However, when I try to make a side blog on the old account, it goes through fine. I vaguely remember reading about how Tumblr saves a URL for you after you delete a side blog. How long does it take for the URL to be up for grabs again?