There are no hard and fast rules, but in Australia, current recommendations are that treatment should be commenced when:
Your CD4 cell count is 350 or below.
AND / OR
You have symptoms of an Aids defining illness.
If you have less than 350 CD4 cells and/or a viral load above 100,000, it is time to seriously consider commencing treatment if you haven’t already begun. It is equally important for you to be "ready" or in the right frame of mind to commence treatment. If you have concerns about starting, discuss all the pros and cons with your doctor, nurse or treatments officer. Once you have taken your first few doses the whole idea becomes far less worrying.
If your doctor recommends treatment, but you decide not to commence, just yet, it’s highly advisable to have regular check-ups, including blood tests, at least every month.
From the Aids Council of NSW