When is the appropriate time to discuss salary expectations?

It's in the candidate's best interest not to discuss compensation until there is an offer on the table. You first need to first get the employer to love you and feel that they've got to have you. Once you've accomplished this you will be in a much better position to negotiate your employment package.

If asked for salary expectation early on in the interview process, you can respond by saying "I'm happy to discuss salary at the appropriate time but for now I'd like to discuss how I can add value to your organization and learn more about the position to determine if it is a good fit. Would that be alright with you?" or "I'm sure that your company pays competitively and I don't anticipate that salary will be an issue."

Another strategy is to turn the question back on the employer and ask a question of your own. For example, you can ask what the salary range is for the position. Once you have this information, you can let the employer know if this range is consistent with what you are looking for without disclosing an actual dollar amount.