With most crafts, you know right away if something works - your earrings don't fall off, your skirt fits, your cookies look and taste great. With making bath & body products, time is an essential component in determining success or failure. I like to give it at least three months - preferably six months - before I consider a product ready for gifting. (Which means any new products I'm making now will see the light of day between return to school and Christmas!).
epic lotion fail? It took 7 weeks for the lotion to separate! I used one regularly in my bedroom - and that bottle was empty by the time I noticed the separation on the workshop storage bottle. When you make a batch of something, leave one bottle aside in a clear container for testing purposes and don't move it. See how the colour changes in the sunlight or heat, see if it becomes clearer (in the case of surfactants) or cloudier, how the consistency changes, and so on.
You need time to see how you and your testers like it. A winter time moisturizer might feel too heavy in the summer, and something that makes your frizzy hair feel great in the low humidity of winter might give you serious frizz in the high humidity of summer. You need time to see if you've chosen the right container for the product - I love the look of cosmo oval bottles, but they aren't sturdy enough for the intense spraying I need for a leave-in conditioner.
Listen to your testers. No one likes to be told their product isn't great, but you can learn some valuable things if you throw your ego out the window and really listen to the critical feedback. One mother told me I needed to make my bath bombs smaller for kids because they have less water in their baths - I didn't even think of that - and I need to know if my body wash is moisturizing for different skin types. It's not easy to hear criticism, but it can be very valuable!
You need time to test it with your preservatives. And remember, not all contamination is obvious. If you're selling, get your products challenge tested at a lab to find out if your preservative is effective (I don't know any labs in your area, but you can find them in the Yellow Pages). If you aren't using preservatives on water based products, don't even think of selling or giving away your products!
You want to test your products in all kinds of surroundings - in the dark of your workshop, in the light of the kitchen, in the heat of your car in the summer, in the humidity of the bathroom, and so on. People may keep your products for years to come, and you want to make sure you aren't using oils that will go off in three months. If this is the case - like the grapeseed oil bath bombs I made on my first try! - make sure you label your products with a really obvious best before date!
You need to test larger batches of products if you're planning to sell. Making 100 and 200 gram batches is fine, but things can change when you increase that size into 1000 or 2000 grams. It might be that every time you made that lotion, you added an extra gram of emulsifier - a big deal at 100 grams, but it means epic lotion fail at 1000 grams. And your little hand mixer or stick blender won't do larger amounts - you might need a paint mixer when you get into the 5000 gram batch size - so you might see some emulsification issues.
I hope I've offered a few ideas for your consideration when it comes to judging whether your products are ready for prime time! Any other suggestions - post them in the comments!