You would use "until then" if there was some event referred to earlier in the conversation that would bring you back together again. You would use "so long" by itself as "goodbye". "Let's meet for lunch tomorrow." "Sure. Until then..." [By the way, nobody knows for sure the origin of "so long" as goodbye. Some think it is a mispronunciation of the Arabic "salaam".] "Have a good trip back to China." "Thanks. So long."

Until then and so long are both casual ways of saying goodbye. So you might say it to your friends when you're going home, but you wouldn't say it to your teacher. For example, you go to a restaurant with your friend. After you eat, you will go to your house and your friend will go to his house. So you won't see each other for a while. But maybe you and your friend plan to go to another restaurant this weekend. "So long" means something like---> until we meet again after a long time...I'll be waiting. Or... I won't see you until we go to that restaurant on Saturday.