Wine will eventually turn into vinegar. So, other than having a really icky taste by itself (unless you like drinking vinegar :)), it isn't bad for your health and you can cook with it.
It probably won't hurt you, but I wouldn't risk it, just in case. Glazes for food containers are unlikely to contain lead, but may have other metals that are harmful in sufficient quantities. These include cadmium, barium, cobalt, strontium, chromium, and in the case of some older orange ceramic, URANIUM.
Blue glaze is probably copper or cobalt based. I found a reference on ScienceDirect about high blood cobalt levels in association with lead poisoning from glazed Greek earthenware. This indicates that cobalt can indeed be leached out of the glaze and absorbed into the body. Cobalt is eliminated rapidly, but is moderately posionous, and copper is problematic in large quantities. Which brings this back to where I started: it won't kill you, and probably won't make you sick, but it's not really worth the risk.
File it in the same category as eating egg salad left out in the heat a few hours?
One other caution: prolonged contact with acid may damage and discolor your formerly handsome crock, in addition to leaching out these metals. To check for this, you can leave white vinegar in the container for a week or so, checking for change in color of the vinegar and the container every day.
Best Answer
There is not really a turning point. The way that wine goes bad is the process of making vinegar. From wikipedia:
The word "vinegar" derives from the Old French vin aigre, meaning "sour wine".
The Canadian government limits things that can be sold as 'vinegar' to something with an acetic acid content of 4-12%, so you could do the technical-bureaucratic thing and wait for the acetic acid to get up to 4%.
If you aren't hung up on technicalities, then you can use it as soon as it gets sour enough. It is done when all the alcohol has been changed to acetic acid, but it is a bit hard to taste since the acetic flavor dominates.