Most Helpful Guy
Different for different people. Often it depends on what all is going on in the lives of the people you are talking about. Work, extended family, health, etc.
Most Helpful Girl
The length of the honeymoon period depends on each couple. How their work schedules line up, any other of their time commitments, how stressed out they generally are, etc., on top of how open both are to some compromise to help make things between them work out.
It's generally said to last 3-4 months before the rose-coloured glasses come off. It doesn't necessarily need to end at all though... my boyfriend and I have been together for 2 years and still feels like we're pretty darn close to being in the honeymoon period. :)