How many days does it take to drink 12 Oz, if it indicates 2 teaspoons every 6 hours?

For any question like this, it's usually helpful to convert into metric measures. A teaspoon is equivalent to 5ml, so 10ml every 6 hours is 40ml per day.

Now we convert 12 Oz to metric (this depends on where you live, as a fluid ounce is different in different countries). I'll assume America because you said Oz instead of Fl. Oz. An American ounce is 29.5735ml so 12 ounces is 354.882ml.

At a rate of 40ml per day, the 354.882ml will be finished towards the end of day nine.