Assuming you make your soylent in the morning, you would want to refrigerate what you don’t drink immediately, or take with you to drink throughout the day. This is per some people who have tried it already, stating the mixture will spoil by the end of the day, otherwise. So, for any soylent with your oils and liquids (water, milk, whatever) mixed in, I would not count on it lasting more than 8-10 hours, refrigerated. Though, be prepared to remix whatever amount you’ve put away as, depending on the ingredients, separation is possible.
Now, for any unmixed soylent (powder), the shelf life would basically be at the whim of whichever ingredient goes bad first. Now, this varies significantly depending on what you are using and what the substance is. You will have to look into what is advertised as the average shelf life of each ingredient in the recipe as well as the best conditions for long term storage, which usually entails something in the realm of an airtight container stored in a cool, dry place.
Hopefully this helps you, at least a little bit. Otherwise, maybe someone has firsthand experience with this recipe and it’s ingredients.
This is actually a good question for people to keep in mind in regards to DIY recipes and their ingredients because people have to make sure they do not order more than that which will survive for the period of time that they have it. So, what I have done for recipes is to finalize all the measurements, calculate how many daily portions are included in the entire container/amount that is set to be ordered, and then compare that to the time frame I find for the shelf life of each ingredient. This ensures I can adjust the amount to be ordered, if needed, to be more or less, depending on if the ingredient will expire before I have the chance to use all of it, or if the amount ordered won’t cover the entire month. You can also lower the amount to be ordered to fit it within a certain time frame, say a month, to simplify things if you want, which I have done for a few ingredients.