You will have 45 minutes to take your citizenship test. This should be more than enough time to finish your test as it only involves 20 questions. Many applicants finish their test in a a very short time.
If you find that you are struggling to complete practice tests in this time you may need ESL/IELTS classes to improve your English language skills.
Migrants and refugees who can speak little or no English can take advantage of the Australian Migrant English Program (AMEP). The AMEP will provide free tuition up to a 'functional level' of English.