1 year, give or take. Everything beyond that you might as well call it something else other than courting

Edit (let me elaborate):
Given that the two of them have reached the courting stage (they know each other to a degree by now), I guess a year is more than enough time to see if you are meant for each other/ a marriage could work out. A year will be a broad enough time frame to experience them and their character through the seasons so to speak, i.e. in times of increased stress at work/school etc.
You will see how they fare through the general ebbs and flows of a year, how they achieve stability through changing times or more generally how they switch up things from the monoton and the mundane.
1 year should give you a glimpse into their life on a microscopic scale, and it gives an idea whether or not you are ready to take a leap of faith and tie the knot or not.