No one knows the answer to this. No one will know it for several hundred years. They have only been commonly kept here in the US since about 1990.
Most reptile grow to adult size relatively quickly and then slow down a lot. I have read that they continue growing, albeit very slowly, for the rest of their lives.
Pretty much "forever" (not really forever, but an exceedingly long time). With certain types of animals, many tortoises seemingly among them, it is not the age itself and associated cellular degradation that appears to do them in, but an external factor like a pathogenic microorganism, a parasite load that is too high, an environmental quirk, or a predator. A combination of these things often exacerbates matters and will act in concert to result in an untimely tortoise demise more quickly than it might occur otherwise. In pet tortoises, I would bet human keepers are the number one root cause of tortoise death. Do you ever wonder why those of us who breed can produce legions of babies and still have a strong demand for more babies? It is because most babies in captivity likely die. Hatchling fragility is one thing, and some do simply fail to thrive, but it usually seems to be a keeper mistake that offs them.