Telephone interviews can be of any duration although experience has shown that there are thresholds after which respondent cooperation and quality of responses decline. We've had the best results when interviews don't exceed 15 minutes. Generally, business interviews are shorter than consumer interviews although both are a function of the topic, types of questions, whether the client is identified, whether respondents are customers or non-customers, and so forth. In some cases, cooperation for lengthy interviews can be increased via pre-recruiting, alert letters, and incentives. Internet and mail approaches are similar because they are both self-administered, allow for longer surveys, and can utilize graphics.
All methodologies have advantages and disadvantages.