That really depends on the event. Competition for the stretch limousines can be fierce for weddings and formals. Weddings are usually booked between 6 to 12 months. If you are getting married in the peak times between spring and Christmas then it is advisable to book as early as you can.
School formals are booked 2 to 3 months in advance. Remember that there will be a lot of demand on the night from your fellow students, so you will need to get in first!
Major sports events like racing day carnivals or state of origins are also very popular for chauffeured limousine hire, so the same rule applies. First in best dressed!