As far as shelf-life goes - shellac has a shelf-life? A can of prepared shellac will become thick, 'muddy,' and eventually turn into a solid crust as the alcohol evaporates off over time, but you can reverse this process at will by adding more alcohol, giving the mix a stir, and allowing some time for the shellac to become fully dissolved again.
This, from the wiki page states what I've always been told about it:
Liquid shellac has a limited shelf life (about 1 year), hence it is sold in dry form for dissolution prior to use. Liquid shellac sold in hardware stores is clearly marked with the production (mixing) date, so the consumer can know whether the shellac inside is still good. Alternatively, old shellac may be tested to see if it is still usable: a few drops on glass should quickly dry to a hard surface. Shellac that remains tacky for a long time is no longer usable. Storage life depends on peak temperature, so refrigeration extends shelf life.