I don't think it's a question of how long someone has trained before they first attempt to trial - like you said, some teams may progress more quickly than others for lots of reasons. For me, I don't want to step into an agility ring (or any venue) until we are proficient in training at a level above what we are competing at. All the nerves, excitement, and extra distractions of a trial tend to take the performance down a notch. Fun runs are also great, if you have them in your area. Though they can't quite duplicate the trial environment, it's about as close as you can get - and you get the opportunity to train in the ring if a situation should come up.
I TOTALLY agree with going to observe a few trials both for yourself and the dog. Make sure you read the rulebook - then, seeing it all in action will answer lots of questions for you. And the more questions you can have answered, the less nervous you'll be leading up to the real trial!