Glen runs valves properly and nothing is pushed beyond its normal sweet spot from what I've seen. Despite the quoted figures, there is no valve 'lifetime' apart from some military spec stuff. It's possible for valves to fail at any time, though much less likely for small signal uses. It will also depend on the make of vale, as most new production is bettered by older stuff, though the provenance is more certain.
For the sort of valves used in the Croft, and the way they're used, you should be looking at probably around 5000 hours before anything starts to decrease in efficiency, though less or more is perfectly possible. Some small signal valves can still be good for a lot longer than that.