For me NRE is basically "the infatuation stage" which was written about briliantly by M. Scott Peck in The Road Less Traveled. 6 to 8 months is the average length, but of course that can vary too. Following the NRE is the phase where you have to deal with shadow stuff and that is usually what ends a relationship. Some of us have gotten so good at keeping the shadow in the attic that it could take 18 to 24 months before the real shadow work takes place. A lot of relationships end there. If you can make it beyond that, you might have something long term!!
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.