I've had most of a opened bag of cement stored in my dry hut, wrapped
in a moisture proof bag, for the past almost 3 years, dipping in as
needed - none of which has been for structural work, just odd bits of
repointing etc.

The only difference has been now it seems to have developed some small
lumps, which disappear in the mixing and it takes much longer to fully
set up.

I've read six months is the limit of useabiity.