Once approved, how much time do I have until I have to move to Australia?

With many temporary visas such as an ETA Visitor Visa, Short Stay Business Visa or Holiday Working Visa you have 1 year (calculated from the date your visa is granted) in order to arrive in Australia. In other cases, such as temporary work visas, you may only have a number of months to enter the country.
Permanent visa holders normally must arrive within one year from the date of their medical test or police certificate, which occur during the processing of your Australian residence visa application.

You will generally be allowed 6 to 12 months to come in Australia.