This is calculated from the date of your approval and is from the date of your completed final medical, you normally have 1 year from this point to make entry. However after the medical you will still need to allow for time for the visa stamping at your nearest consulate office and therefore this maybe a further 3-6 months.

You will generally be left with 6 to 12 months to land in Australia. The exception is for temporary visas, such as work visas, in which case you may only have a number of months to enter the country which will be stated in the visa validity on the stamp in your passport.