The main idea of Distant learning such as OTEN is that you have the flexibility in learning you may not receive from a TAFE campus. You are able to enrol at any time throughout the year, although you still need to re-enrol every 12 months. And also, with doing courses via OTEN the normal holiday periods may delay some results/feedback due to the teachers being on holidays also. The time frames on most courses will have a maximum time although everybody works at a different pace- thus the flexibility- and therefore will complete the course at a different rate.
Hope this helps.