It definitely depends on the product. Like SMichelle said, I usually feel like I can review lingerie after wearing it once--although if it's lingerie that's supposed to be more functional than sexy, I wait until I've had a chance to sleep in it, wear it out to a club, etc to see how it holds up.
Toys, on the other hand, I use at least twice to make sure it wasn't a fluke. For beauty products, I wait a few weeks to see if I'm getting the desired effects.