You can stay in Australia for a maximum of 3 months on each visit. 

After your ETA is granted you are able to enter and leave Australia as many times as you need to during a 12 month period, from the date the ETA is granted or until the expiry date of your passport, whichever is earlier.

If you are planning to be in Australia for longer than three months, you will need to apply for a visa that allows for a longer stay in Australia.