How long can a friends with benefits arrangement last?

This guy and I have been FWB for about 6 years now. We have both moved to different cities and I am going to see him this month. The last time I saw him was when he came to my city a year ago. My question is, how long can this last. I don't even know if we're dating or just really long term FB. Is there such a thing as a 6 yr. FB?