LOL! You're even lucky to see 4 months.
Actually, it varies. I think it's anything from 2 months to 1 year (rarely longer). The closer you are to each other physically, the less honeymoon period you'll enjoy. The more distant you're from each other, the longer your honeymoon period.
It's so because when you're distant, you can make believe and think it's all someone else's fault . . . someone who objects to your relationship, for instance. You can also make believe that if you get physically closer, things will get so much tighter between you two. At a distance, it kind of resembles a forbidden love 'cuz you're both struggling to stay closer and are both aiming at reaching each other physically.
On the other hand, those who live with each other everyday can see every single dirt of the other party. It's no longer the fun discussions. It's who's going to do what. Why do you fart in my face, etc. You didn't buy me a gift for my b-day, etc. You get so used to someone so fast and then it doesn't seem that fun anymore. Of course it's fun and lasts for those who are responsible and make effort to work things out. 
Edited to say "2 months".