What does "schon" mean in this sentence?
What is the purpose of schon in the sentence, "Ich weiß nicht, wie lang haben wir uns schon nicht mehr gesehen"
In general "schon" mean "already". In this sentence it has the function to stress that in the opinion of the speaker it is quite a long time ago that the two persons met each other.
"schon" in this sentence functions as a modal particle. It is used to indicate how the speaker thinks that the content of the sentence relates to the participants' common knowledge.
In this case, schon is equivalent to the afrikaans modal particle "darem" (Sorry, this is how I decipher German particles. :) Don't judge me). "Darem" does not meaning anything in itself in this sentence; however, it adds a sort of urgency to the fact that the two parties haven't seen each other in a long time.
It simply underlines the fact that the two haven't seen each other in a long time and maybe indicates that there is a possible intention to reunite in the possible future (otherwise there is no real reason to use it).
"Man, we haven't seen each other in such a loooonnng time". loooonnng = long, but with more urgency. This is almost the same as schon.