How long can I go with out taking Levothyroxine?

I am on a high dosage for hypo thyrodism (300 mcg). I have been on this medication for 12 years, and have had my dosage increased several times. I usually do not go with out taking it as it makes my moods out of control and makes me very tired. But, I ran out of my prescription and need to refill it which costs me $20 even with my insurance. I don't have any money until I get paid next week, and it will be 3 weeks with out taking it. Is this safe?