Mortar should be mixed in batches that are small enough to use within a few hours. If you mix a batch that is too large for immediate use, there are ways of storing it securely for use the next day or even later. The secret to keeping mortar from hardening and being ruined is to separate it from air.
Keep Mortar for a Short Time
Scrape the mortar off the sides of the container in which you have mixed it, compacting it in the bottom of the container so it doesn't have bits and pieces running up the sides.
Press a piece of plastic sheeting into the container so it contacts the top of the mortar. Press the plastic down all around the edges so that it covers the entire top of the mortar.
Place the container in a cool location that is out of direct sunlight. Mortar will keep like this for several days without hardening.
Keep Mortar for a Long Time
Place the mortar into a plastic bucket that has an airtight lid. Consolidate the mortar in a compact clump in the container.
Pour water into the container until there is an inch of water sitting on top of the mortar.
Put the lid onto the container and seal it. The water and the lid both act as barriers to keep oxygen away from the mortar.
Store the container in an out-of-the-way place that doesn't get too hot or cold, and is protected from direct sunlight. Mortar stored in this way will last for months without hardening.