G'day again,
The largest (and therefore, the potentially longest)
mosses I know of are the 'spagnums', as in ancient bogs and now used in
potting mixes. It is my understanding that theoretically, (and presuming
good conditions),they can grow forever from the leading shoot, even as their
'roots' compost into history. They were commonly used in both horticulture
and medicine for their anti-septic qualities, hence the bogs and the ancient
artifacts that are found there. I personally have seen 'branches' of spagnum
moss over a meter long, and I then cut it up, dried it, and added it to
orchid and fern potting mixes with good results. While the classic spagnum
bogs are in Europe, I understand there are several in Aus. that National
Parks has no intention of advertising.

China
Wingham
NSW