Welcome to AFF!
There’s no absolute hard and fast rule for getting the cheapest flights. As a general rule, however, booking months before the flight will normally get you cheaper prices than booking days or weeks before the flight. So planning ahead, and a willingness to be a little flexible, is a pretty good formula for getting the cheapest flights possible.
The first step is to monitor Jetstar’s prices on the routes you want to fly. Have a look at what they cost if you book 11 months ahead, or 6 months ahead, or 3 months ahead, or 3 weeks ahead. That will give you a very good idea of the “normal” price of those flights. Be aware that flights will generally cost more during school holidays.
Once you have a good idea of what the flights would normally cost, be on the lookout for sales and/or promotions (I assume you can sign up to receive notifications of these from Jetstar). Then when those promotions (such as the Friday sales mentioned by
@AIRwin above) come along, and if the prices are cheaper than the “normal” cost of the flights, that’s the time to grab them.
I generally fly Virgin but over the years I’ve saved many thousands of dollars by following that strategy of booking many months before my flights, during sales and promotions. I’m sure Jetstar is the same.