Preferably forever. A confirmed bachlorette here.
I think people live together AT least 5 years before deciding if they want to have a future together--whether or not that includes marriage.
Dating gives no indicator as to what a person is like day-to-day, hour-to-hour.
Living with someone is the only way one can tell if the other person is a person one would be able to live with long-term.
One cannot tell how well another is at helping around the house if they do not live with that person. Many women I know have gotten trapped into sub-slavery because they haven't lived with a person --or lived with a person long enough--to realize that all the household chores,cooking, & child care will fall on them.
Cute little personality quirks become annoying habits when one has to live with such things. Only living with someone for a while can determine if one can learn to ignore these habits.
Also, you'll be able to find out if you are involved with an ambitious person or a couch potato. You will be able to find out if you don't mind your partner staring at a screen 70% of her/his life, or playing video games instead of going out for a jog or hike with you--if that's what you're into.
And why 5 years? Well, after 5 years the honeymoon is over and day-to-day doldrums set in. THAT'S when you find out if you're capable of putting up with the other person.
DON'T GET TRAPPED. Happy endings happen only on the big screen, not in reality. Reality is fraught with difficulties and conflicts and ever-changing dynamics. The person you live with today will be a much different person 2 decades from now. This is how it's suppose to be. Life is fluid.