4 months, two weeks and three days.
But seriously, in my experience it is usually some time after the third date and by the fourth month that you both lose interest in other things (other guys) and are both thinking mostly about each other. When then happens to both of you, and you can talk about that easily, you will know. It comes about quite naturally.
If you force yourself into that frame of mind, you will not want to sustain it. That would not be honest with yourself or with him.
A rare few relationships can remain open from the start, but from my observation those are not as deeply committed or as satisfying as exclusives and do not last as long (decades or lifetimes.)
And it is not unusual for exclusives to open up again to some degree after several years.
Hope my observations are useful.