Are bacteria necessary for an individual's life, or, how long could a person live without bacteria?
You might need to demote your single-celled 'lords' to 'squires'. They're not essential to an individual's life. You wouldn't die (dispensing with the "how" right off the bat.) You'd be just fine if no bacteria reentered your body. Your fecal output would be greater; you would derive somewhat less nutrition from your food, you would need to take vitamin K, and there would be an adjustment period, but theoretically, you'd be just fine. You might even be healthier.

Your hypothetical is impossible, of course, because experimentation on humans is unethical. However, it might surprise you to learn that there are colonies of germ-free animals (also called gnotobiotic) used to study the effect of certain bacteria on animals.

The derivation of such subjects was initially fairly gruesome.


  The process of engineering biology and machine began by removing intact uteruses from near- to full-term pregnant animals within a purpose-built germ-free "surgical isolator." The uterus was subsequently passed through various disinfection procedures, involving total immersion in germicide-filled "dunk tanks," before the progeny were surgically released and hand reared within a second microbially sterile isolator.


These animals are costly to keep, as you can imagine. The originals must be delivered by Cesarean section, and housed and raised in sterile conditions. But once you can do that, they can multiply. 


  To start a germ-free colony, one must remove a young animal from its mother’s womb through a careful surgical procedure to avoid exposing it to the microorganisms in the mother’s vagina and skin. Then, the animal is raised in a sterile cage and only exposed to food, water, and other equipment that has also been sterilized. On a weekly basis—or more often—a technician swabs cages and animal feces to ensure that no bacteria have contaminated the sterile housing. Once the colony has been created, it becomes easier to rear new germ-free animals; a germ-free mother can give birth naturally without exposing her newborns to any bacteria.


By the 1950s, researchers were rearing germ-free mice, rats, guinea pigs, and chicks inside sterile stainless steel (and later, plastic) housings. Germ-free animals are especially useful in researching, among other things, diabetes, auto-immune diseases, obesity, genetic engineering, cancer, immunology, nutrition, and the effects of normal intestinal flora on health.

Vitamin K must be added to their diets, and it is important to sterilize their food without destroying nutrients, but otherwise they do well in their environments. Their feces, of course, are sterile. Everything is microbe-free. They also have no "normal" antibodies.

Since generations of germ-free animals can exist and multiply, it is not only possible but quite likely that germ-free humans can exist as well.

Which is not to say that gut (and skin, etc.) microbiomes are unimportant. If you release a germ-free animal into a normal environment, it can die if it picks up pathogenic bacteria before more normal gut flora. To do this safely, a germ-free animal is first given a cocktail of bacteria to colonize the gut. After a while, they can be in a normal environment without problems. But your hypothetical human wouldn't even have this problem.

Germ-Free Mice
Gnotobiotics
Technology Highlights：ESTABLISHMENT OF THE FIRST GERMFREE MOUSE AND RAT COLONIES IN TAIWAN
"Life in a Germ-Free World":
Isolating Life from the Laboratory Animal to the Bubble Boy