Hi welcome to the forum.
When it stops blowing cold are you sure the compressor is still engaged? Borderline gas pressure/level can cause compressor clutch engagement to be hit and miss. Or maybe it runs but the cooling performance is not enough to keep the condenser radiator cold for long. These are just suggestions. i am not an expert
Have you owned the car long enough to know that any previous regassing has normally lasted a long time - years rather than months? If you bought it fairly recently then maybe it was regassed just before you bought it and the periods between you needing to get it regassed, and again having problems might indicate its leaking.
I'd go back to whoever regassed it. They may do a pressure check free of charge,to disprove any responsibility, or in the hope of repair work.
In view of the age of the car maybe there are DIY leak fix treatments and recharge options available in Oz that might work for any slow leaks.