This depends on the visa! It also depends on how busy the Departmental office is at the time. Visitor visas (when appropriate) may only take a few weeks. Spouse or fiancee visas 3 – 6 months. Permanent visas for temporary workers in Australia, between 4 and 7 months. Generally the better-prepared the application, the quicker it takes.
Jeff Harvie is a professional Migration Agent, registered with the Migration Agents Registration Authority, a statutory body set up by the Australian Government to regulate the standards of the profession, and to give protection to visa applicant clients.