Visa if a student

How long does it take to apply for the student extendable visas once you have filled out the forms to enroll in language centre/school?

I am right in saying they last a total of 6 months? 3 x 2 months?

Also I heard you have to have an attendence record of at least 80% or they will cancel the visa?

And last question, what is the total amount of time you have to study per week to be qualify for this type of visa?

New Guy