Talk. To. A. Lawyer.
This could get sticky depending on the company. By saying "you have xx days to fix this before I announce the exploit", you are basically saying "do what I expect, or I will cause you lots of grief".
The other issue is, how did you discover this? Were you using the site 'normally', or did you see the potential for the hole and decide to see if it worked? This is very important to keep in mind, especially if you are considering setting a time limit to fix the issue. I'm not sure what the laws say where you live, so please, talk to someone who does.
You might end up with their thanks, some cash for entering into a NDA (you did, after all, browse the admin interface) and you might get some credit in the security industry. But, be very, very careful and do try and seek the advice of an attorney.