"We could only stay out until 11 P.M."
How long are you going to wait for an answer? Until 8pm? Until midnight? Until next Tuesday? No, "Until." is not a sentence.
'Is until tomorrow' is a fragment, because it has no real subject. You can make it a sentence: What Is until tomorrow?
It is an imperative sentence.
Example sentence - They would want to wait until they sky looked white too.
I will wait until you are done cleaning your room
I play outside until it gets dark!
until you eat your vegetables
Don't come back until 5 pm.
Waited
You is the subject in that sentence. The subject is whois performing the action.
this sentence is going to be continueing forever until i decide to end it.
Example sentence - The painting appeared dimensional until you looked closer.