Q:

What kind of grammatical structure is "all the time"?

You are meeting friends at the cinema all the time.

Is "all the time" a preposition structure or a noun structure?
"all the time" means always do something.
I think "all the time" is a preposition structure which omits the preposition "for". If "all the time" is a noun structure, how can it modify other parts of the sentence? Can a noun structure function as an adverb?

A:

I would call it an adverbial structure. In your example, it describes when you are meeting friends at the cinema.
Consider: 

You are making new friends daily.

There, daily is an adverb. In your sentence, the phrase all the time is functioning the same way.

