What do we actually do during stocktake?
Some libraries call it shelf-check or inspection week(s), but essentially do what a lot of libraries try to do once a year: we check that every book we believe should be on the shelf is on the shelf. This means that we have usually two teams of two librarians starting at different shelves - one calling out the classmark on the shelf, the other team member checking a printed out list.
Why does it take two weeks?
We usually need about 7-8 working days for the actual check, then we need to spend some time on tidying up the records (and maybe even investigating some problems); often, we also do a book move during this time. The last day of stocktake week has traditionally be the August Bank Holiday Monday, when we do not work.