Preparing the shake when you are ready to drink it is the simplest option. If you prepare it ahead, follow any product-specific storage directions and refrigerate it promptly in a clean, covered container.
As a general food-safety rule, do not leave a prepared shake at room temperature for more than two hours, or one hour above 90°F (32°C). Use stricter instructions on your pack if given. Do not assume it remains suitable for 24 hours without product-specific guidance. When in doubt about storage, discard it.