It lasts longer as stock solution.
You should always mix up the whole volume of the package. If you do not need the whole volume right away and don't have an "Air-Evac" accordion style bottle, one method of keeping it longer is to divide the mixed stock solution into smaller, full bottles.
It's a good idea to leave the stock solution to settle overnight before use to be sure it's completely dissolved but not absolutely necessary.
If you dilute the stock to make the working solution, you do not need to leave the diluted solution to sit overnight. The stock solution should mix quickly and completely with water.