Many of the brewer supply company kits (such as those from Northern Brewer or Austin Homebrew Supplies) tell you to wait at least two weeks before testing; they know you'll be back to order more if they help you make a good beer! They make much more on their own kits than if they were to sell you a Brewer's Best kit.
Different beers behave differently in the bottle, such is life. My Extra Pale Ale (from NB) was good after two weeks, clarified after at least three, and then started to lose aroma and gain bitterness after two months. Light ales really should be consumed fairly quickly, but that's offset by the fact that they temper quickly in the bottle. My Scottish 70 Shilling Ale, though, is darker, less bitter, so has grown better in the bottle after three months (very clear, nice head retention, no odd aftertastes).
The amount of priming sugar added really makes a difference, as well. The standard kit will say add 5oz of corn sugar to five gallons of brew, but that number should be adjusted to the type of beer. 4.25oz for my IPA was just right, and 4oz for my 70/- seems perfect.
How you pour makes a diff, too. The old way of "sliding it down the side of the glass" doesn't make much sense with ales, as you want that frothy head to build a bit in the glass in order to make the aroma of the hops come out of suspension. Halfway into the pour, pick up the bottle a bit and let it hit the center of the glass. Compare the smell to one you've carefully poured down the side of the glass; the more frothy pour will wake up your tastebuds for sure!
Anyway, it's going to be drinkable at most points in the bottle. The subtle differences really start showing after different times depending on the style of beer you've brewed. There's no hard-and-fast rule, but that's what the kit directions tend to show. Too bad.
