How long a human being could live with stem cells?
Science-based and the science-fiction tag, hmm.
If the science is right and with some restrictions, perhaps forever barring physical assault or accident.
A human normaly ages such that each time it's cells reproduce to replace aging injured ones, the ends of the helices, the telomeres, allow the chromosomes to unravel, copy themselves, then ravel up again - each time a part of this chemical tail - the telomere gets broken and lost. When the cell has reproduced so many times the tail disappears, the DNA unravels a little and cell metabolism is disrupted, copy errors multiply like an ancient hard drive, the cells fail, the organism finaly reaches the end.
The treatment would need to begin young. The reengineered pluripotent cells would be incorporated early in the child's developement, possibly before birth. They would incorporate themselves within every tissue, multiplying as the child grows taking the shape of a human adult finaly - able to reproduce like any young adult at the peak of their potency.
Gradualy the tissues would age and the adult human would start slowley imperceptably to lose potency, not quite as strong or as fast or as fierce in their lust for life. Time demands a toll.
The stem cells, reengineered pluripotent cells as they are, have been carefully engineered from not just human DNA but, learning a lesson from some of our most distant primitive cousins the jellyfish are functionally immortal - but for them to live young forever there is a toll.
Preparation: A place to rest and lie undisturbed, attended buy one faithfull companion or an automatic machine to be activated at the right time.
Stage 1: Age shows, the body is weary, it must rest. A period of rest and starvation. The cells begin to revert from their tissues, bones soften, muscle becomes stem cell, sinue, fat, marrow organ and skin all become stem cell again, leaving barely enough to sustain the most basic of functions - respiration, keeping the precious brain fed and cushioned safe. The body almost dissolves within the sack of dead skin, settling outwards like a human suit filled with a soft gell, until all is still.
Stage 2: The feeding tube is activated to give: Sustenance, nutrients must be absorbed, cells transforming and growing in strength once again, from the mass of undifferentiated flesh the shape of a human can once again be discerned, taking shape, moving slowly, tissues shifting and coalescing, settling into it's new form. The sack of dead skin finaly starts to split and the creature arrouses and arises, shedding the face of it's aged self adopting a familiar face from it's own past, young once again. All that remains on the floor - eerie dead skin, no flesh and some little putrescence of waste.
It's organs and glands are young again, the blood vessles in brain and heart new - stripped of the plaque of any excesses of the previous cycle. It's skin free of scars and blemishes, it's eyes bright and quick. Copy errors are corrected in the chromosomes, the helices are raveled up and the telomeres are long and strong once again.
How long to live? Who knows, as long as the toll is paid every few years faithfully and the process is allowed to finish uninterrupted. How long - long enough to forget the begining or perhaps that there even was a begining.