You should answer a question as soon as it helps YOU to understand the concept, solve the question.
The idea behind the "accepted answer" is not to indicate which answer is the best solution (that's what voting is for), but to indicate which answer helped the OP (original poster=question asker) specifically.
Unfortunately, some users consider accepting an answer as the functional equivalent of closing a trouble ticket — "thank you, problem solved; nothing further needed here." This is not the case. Overall, you should try to hold off on accepting an answer as long as you can manage. It shows that questions are still open to input and encourages further collaboration and improvement. Incidentally, you can (and should) add new answers to a question with an already accepted answer if you think something is missing in the prior answers.
Also note that the 2.5 answers per question criterion is not the most important requirement. If this site succeeds to build a nontrivial repository of knowledge, it can't really fail. Read more about beta site evaluations here.
I recommend also reading these help center articles: