What is a 'classic paper', and when do we refer to a work as a 'classic work'?
This isn't something that is decided based on objective measures - it's a statement of the author's opinion that history has shown the paper to be very important.

If you, in your professional opinion, believe that the paper has had a major influence on an area of research, for a considerable period of time, then you could call it "classic".  If you don't believe that, or you're not sure, or you don't think you have enough experience to judge, then don't use the word.