Long story short, it’s pretty impossible to give you an accurate answer to this, but I’ll try to help you determine a ballpark. Probably at least a year or two, but also “it depends, x1000”.
For 100% oil-based concoctions (lip balms, body butters, massage oils, etc.)
Because these things contain no water, we’re only concerned about rancidity—that is, the oils that make up the product oxidizing. You’ll usually get at least a year out of a 100% oil-based product, but that can be impacted by a few factors:
- Storage. Cool & dark = longer shelf life. Warm & bright = shorter shelf life. (Store un-started projects, like extra lip balms, in the fridge before use for the longest shelf life.)
- Ingredient freshness. Fresher ingredients = longer shelf life.
- Ingredient shelf life. Some oils have longer shelf lives than others and will shorten the shelf life of your entire product. Check with your supplier to determine the shelf lives of your carrier oils (good suppliers should include a “best before” date on the carrier oils they sell). Examples of carrier oils with short shelf lives (generally less than a year) include flaxseed oil and evening primrose oil.
- Added antioxidants. Adding an antioxidant like Vitamin E or rosemary oil will help extend the shelf life of 100% oil-based products. Antioxidants are not preservatives!
- Contamination. If you get water in a supposed-to-be-anhydrous concoction, it joins the second category in this list and the shelf life will shorten drastically.
For concoctions that contain water (lotions, body mists, creams, etc.)
Because of the presence of water, these projects can and will quickly sprout mould, fungus, and other gross things quite promptly without the inclusion of a broad spectrum preservative. Even with a broad spectrum preservative, these things can eventually spoil.
As with food, I would recommend that you are as clean as possible when making, avoid making more than you can use in a couple of months, and watch for spoilage (changes in colour, texture, scent, or mould population).
Here are some factors that will impact shelf life:
- Inclusion of a preservative. If the formulation includes water (and the water will stay in the recipe and not be dried out of it) and is more than a single-use project (like a face mask), you need to include a preservative. Read this for more info.
- The formulation itself. Some formulations are harder to preserve than others. The more bug food you include (aloe vera, botanicals, clay, honey, etc.), the faster the product can spoil. Preservatives will help extend that shelf life, but that shelf life is unlikely to be indefinite.
- Freshness of ingredients. As with all things, fresher ingredients last longer. If you make a stew with nearly rotten meat, that stew will spoil faster than stew made with fresh meat. It is completely possible for an oil in your lotion to oxidize (go rancid) before bacterial spoilage sets in; short-lived oils like hemp seed and flax seed are definite candidates!
- Storage. Cool & dark = longer shelf life. Warm & bright = shorter shelf life.
- Cleanliness. Keep your kitchen and utensils as clean as possible while you’re making the project, and avoid contaminating it while using it (consider choosing pump-top bottles instead of open jars for lotions so you aren’t dipping dirty fingers into your final product).
So… how long will your product last? Sorry, but there are way too many variables to answer that question accurately. That said, that is for how we made (and stored) the formulation. Did you make a substitution or four? Did you package your product in a jar, while I used a pump-top bottle? Are you dipping your hands into your product directly after playing with your dog, contaminating it? Do you live somewhere drastically warmer? Did you use mineral-rich well water instead of distilled water? All of these factors (and many more) can impact the shelf life of your product. With all these variables, it is flat-out impossible for me to give you an exact shelf life. Probably at least a year (but also please don’t spit in your formulations). Please read this for more information.
Trying to figure out how much preservative to add to your final product? I made a handy-dandy preservative calculator that you can use here.
For concoctions that contain water, but are dried out (bath bombs, clay bars, etc.)
If you’re making something that has water in it, but the water will be evaporated off promptly, you often don’t need a preservative. For something like bath bombs, they are dry for the vast majority of their life and are then used up all at once, so no preservative is needed and the shelf life should be more or less indefinite, assuming you keep it dry (if you live somewhere humid consider sealing the bath bombs in an air-tight bag).
For projects that will be continually wetted and allowed to re-dry, like a clay bar… this one is a bit tricky and depends on where you live. and while that’s not always a flawless indicator of best practices, one can assume that the company selling the bar does not want to sell a product that will sprout mould immediately. If you live somewhere humid, this may not be the case, and a broad-spectrum preservative may be advisable.
Thanks to its high pH and low water content, bar soap should last for years. Using a higher superfat can lead to spoilage or rancidity (too much unsaponified fat can go rancid in the bar).
Liquid soap paste will have a similarly long shelf life. Once diluted with additional water the pH should still be high enough to keep it from spoiling, but the final pH obviously depends on how diluted it is (more water = lower pH). You can also impact shelf life by adding lots of delicious bacteria goodies to your liquid soap (botanicals, clay, unsaponified oils, honey, aloe vera, etc.)—too much will impact the shelf life of your liquid soap. You can try adding a preservative if you are finding your liquid soap is spoiling quickly, but few are effective in high pH would be your best bet.