How long should one take to decide upon a life partner?
I'd answer this question differently in different moods. In the mood of the moment, the answer is, as little as possible. Take only as much time as you need to figure out that there isn't something very fundamental lacking, such as total lack of ability to make decent conversation, or that he has no income and no means and/or inclination of making it in the near future, or total lack of respect for women.
If you stretch it out for long, one of two things can happen. Either you discover that you were right in initially thinking that this is the right person for you, or you figure out that you were wrong. If you were right, well, you might as well have taken the big step a while back. If you discover you were wrong, after spending a really long time with the other person, you know what happens? You start thinking that, since you've spent so much time and energy in nurturing this relationship, you might as well spend a little more and make it work. Then there comes a time when you're just hanging on to a relationship that you know is meaningless, but won't admit it to be meaningless. You'll just hang on to it for fear of the consequences of letting go. What if you let go of this, and never again find someone who is even half as good? You know you'll be happier single than you are in this relationship, but you won't let go because of social pressures to be in a relationship.
But you know what? We all have this incredible capability to make adjustments. Once we decide that we have found our soul mate, it becomes pretty easy to agree on the significant things in life, and to agree to disagree gracefully on the others. There is no disagreement that cannot be resolved in a mature way, once you put your mind to it.
That's just my mood of the moment dictating my thought process. Maybe I will express a different opinion on this in a different sort of mood.