There is no tidy number, and the reason is more interesting than a number would be. A moss does not live out a fixed span the way an oak or a mouse does; a cushion grows from its tips while its oldest parts die away underneath, so the clump persists as a colony for years on end without any one shoot lasting long at all. Ask how long a patch of moss lives and the useful answer turns on conditions rather than years: it carries on as long as the damp and shade hold, which is frequently a very long time.
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A cushion that renews itself
Picture a moss cushion in cross-section. The green, growing surface is only the top few millimetres; below it the shoots pale, brown and eventually rot down, in bog mosses turning to the peat that keeps the surface afloat. Growth is at the tip, decay at the base, and the two run in balance, so the cushion stays much the same size while its living tissue is forever being renewed. This is why asking about the age of a moss is a little like asking the age of a lawn or a hedge: what stands in front of you is a population of shoots of every age rather than a single organism ticking towards an end. A well-placed clump can hold its ground for decades on the same rock or bank.
The single shoot, and the whole clone
Individual shoots are another matter, and generally short-lived, a few years of upward growth before they are left behind at the base. But mosses spread as much by breaking and re-rooting as by spores: a fragment carried off and settled elsewhere is genetically the same plant, so a single individual, in the strict sense of one genetic clone, can extend across a bank in scattered pieces and carry on far beyond the life of any shoot within it. Where a species almost never sets capsules in a given place, as several common ones barely do in Britain, nearly all its spread is this kind of cloning, and the same clone may occupy a spot for a very long stretch indeed.
How long dried out?
Much of a moss's apparent lifespan is spent not growing at all. Mosses are poikilohydric, meaning their water content simply follows the air around them, so when it dries they dry too, shrivel and switch off, waiting rather than dying. That dormant state buys them time out of all proportion to their size. Wet a grey, crisp cushion that has sat through a rainless month and it can flush green within minutes; many species endure whole seasons desiccated, and some shrug off years of it. Herbarium specimens kept dry for decades have occasionally resumed growth when moistened. A patch that looks stone dead in August, in other words, may simply be paused, a point worth holding on to before you write moss off, and one picked up again in reviving dried moss.
Centuries in the ice
The dormancy has a spectacular far end. In 2014 a team working on Signy Island in the Antarctic drilled into a frozen moss bank, thawed a core, and coaxed fresh growth from Chorisodontium aciphyllum that radiocarbon dating placed at around 1,500 years old: moss that had greened last in the age of the early medieval kingdoms, revived in a laboratory. A year earlier, researchers in the Canadian Arctic had watched blackened moss emerge from beneath a retreating glacier on Ellesmere Island, plants entombed roughly 400 years before during the Little Ice Age, and found some of it able to regrow. These are hardly ordinary lifespans, of course; they are extreme demonstrations of the same trick a garden moss uses when it dries out for a fortnight. The life in them had been suspended rather than spent.
So what should you expect?
For the moss on your wall, roof or terrarium floor, the practical picture is this. A healthy clump, left with the damp and shade it likes, should persist for years and quietly renew itself, thickening rather than ageing out. It will pass through dry spells looking dead and recover each time water returns. What actually ends a patch of moss is almost never old age; it is a change it cannot ride out, a wall repointed and dried, a canopy opened to hard sun, a lawn scarified away, or a terrarium that clouds and rots. Keep the conditions right and steady, and moss is one of the more enduring things you can grow, far more so than its fragile look suggests. How quickly it fills in, as opposed to how long it lasts, is taken up in how fast does moss grow.