NRE lasts about 6 months, but some here will admit to it being longer. I myself have always found a change in the relationship around 2 months ... Still in the grip of NRE, but more focussed on emotional aspects rather than the physical ones at the outset.
There is a thread somewhere, that discusses the potential to be addicted to NRE. While some will argue over whether hopping from NRE to NRE is poly, I don't understand how this is infidelity, unless the poly partners were not aware of each other.
Personally, I would be tempted to call that sort of hopping more of an open relationship, but mono people can do the same thing and no one will argue as to whether or not they're still mono! So, no judging from me if someone wants to call that their poly.
The other possibility is that the hopping person really is looking for another long term partner, and simply can't see through NRE. They must wait for it to go away in order to determine whether or not that relationship will be successful. Every relationship fails until one (or several) don't, you know?