Tobias has a good answer, but I want to add more information.
what would happen to the juice?
Knowing that grape skin contains wild yeast, it is likely to ferment on its own, the temperature seems ideal for that (75°F or 23°C).
Would the juice turn to vinegar? Wine? Spoiled juice?
The first thing to happen is likely fermentation. After the end of fermentation (about a week or two) the wine has to be protected from air to avoid spoilage. Adam, mentionned that it is covered, but how air thight is the cover? Bottling and refrigeration would help preserve it better at that point. Acetobacter is a bacteria that converts alcohol using oxygen to acetic acid (vinegar), so this means alcohol and oxygen have to be present for this to occur (and the presence of the bateria of course). So air tight preservation is the key for protection.
If wine, what would be the alcoholic content and would there be a point at which fermentation would end?
The usual way to measure alcohol in winemaking, is to use an hydrometer to mesure the original gravity, the final gravity and calculate it from there. However, if you didn't measure the original gravity, you can use a vinometer to measure the alcohol content of a finished wine. Most yeasts will ferment the wine dry (ie converting all sugar content) which could be around 11% depending on the grapes sugar content, but without measuring the sugar content, this is only an approximation.