The maximum time you are allowed to park in most metered spaces is 2 hours (120 minutes), even if you have paid to be there longer. This includes returning to your vehicle and paying for another 2 hours. This time limit is displayed on road signs and/or parking machines (P120) and parking machines will not let you pay for more than 2 hours at a time.
The time limit starts from when your vehicle enters the parking space, not from when you make payment.
If you have received an infringement and you think your vehicle was not parked for more than 120 minutes, then you should appeal. However, if your vehicle was parked for longer than the 120 minute time restriction, then regardless of what you have paid, the infringement is valid.