They are best kept in a cool dry place out of direct sunlight and safe from young children and pets. They will last up to 12 months if stored with care but may weaken with age, so don't keep too many. If you keep them under a sink or in a garden shed use something waterproof e.g. a plastic bag, don't let them get damp or mouldy.
The green garden waste sacks will start biodegrading on contact with organic waste but will remain strong for a week or two. Don't use the garden waste sacks too soon before collection day. The food caddy liners will remain strong for a few weeks.
It helps your collection crews, and is better for composting, if your waste is fresh. Please don't miss a collection just because you haven't much to put out.