I don't know what to say other than to repeat that for this pale ale I just bottled, it tasted "fine" to me at 2 weeks. Seems to me that, since there's no harm in letting it sit in the primary a little longer, it's worth it when you are just starting out to let it go longer than your own "common sense" seems to tell you it should. If you taste it again and there is no difference, no harm done. If you taste it again and there is a world of difference, then you've learned something.
Since patience is probably the #1 problem new brewers struggle with, it seems to me that setting a not-before date for your first few brews is a good idea. I know I'm glad I did -- if I hadn't, I'd have a pale ale bottle conditioning right now that wasn't as good as it could have been.
One more angle from which to contemplate this: Taste is (obviously) subjective and can be influenced by all sorts of other things going on in our minds. If it is your first or second beer and you are really eager to get it bottled and carbed so you can finally taste the finished product, you are going to be subconsciously influenced to give it a thumbs up. Maybe you'll rationalize, "Well, it's just flat, of course it doesn't taste delicious, when have I ever tasted a warm flat beer and said, 'Mmmm, yummy!'?" (That's what I was telling myself on day 14) Or maybe you'll lie to yourself, "Hmm, not as good as I would like, but this is my first batch so of course it's not perfect. This is probably as good as it's going to get."
YMMV, but I'm really glad I set a not-before date of 21 days in primary for my second batch.
Edit: I figured it was implied, but obviously a hard not-after date would be a bad idea, since you can hurt your beer (or even yourself, via bottle bombs!) by racking or bottling beer too soon. If observation and "common sense" tells you to wait longer than your target date, obviously you should. This is probably a moot point anyway, though, since how many of us were brewing our first batch and said, "Well geez, I really wish I didn't have to taste my first brew ever so soon, but the hydrometer says I need to bottle now, so I guess I'll just have to put up with it being ready to drink sooner than I wanted." Yeah, right...