I never noticed a change in cactus myself. Wizard Labs has a new flavor line of their own, The Flavour Wizard, and I have a sample of their cactus that I haven't tried yet.
It's a vague guideline, but as far as the rule of aging goes - fruits are pretty much shake and vape, very few require steeping and even then it's minimal. Creams almost always require aging, usually a week or two, and custards are seldom good before two weeks to a month. The richer the custard, the longer the steep. I'm aging 250ml of Kreed's Custard right now that has to sit for two months to taste as it's intended. And tobaccos are generally two to four weeks but like custard, the longer you wait the better it is.