The course has changed since I enrolled, and I think you have 6 months to do the units in. You have to do the Language and Learning Unit first. It took me quite a while to do that unit but because i spent too much time on the tutorials! The computer unit I did took me 4 months to do (again could have done it quicker but was focusing more on the language and leanring). I've only just started the bio unit , but planning to get it finished by August.

TAFE Open Learning Program guide - TAFE Open Learning

Check out the information here. Let me know if you have any more questions.