@Shanus, 90 days sounds fair for judging how effective a skincare or bath & body product works on your skin. I agree, adverse reactions such as itching, burning and breakouts, irritation etc. are immediate, and one use is enough to know it's not going to work out.
I think makeup is a category where you know as soon as it's on whether or not it's right. We aren't fooled too often in the store, but when ordering from TV or online, occasional mistakes are inevitable -- even with expert makeup divas.