Bismillah

Thank you for your question. There are a number of responses to this question as the question is usually raised with a number of presuppositions. The Justice of God is not measured by the benchmark mentioned in the question as what it means for God to be Just is that He accords everything and everyone their rightful place. Justice is not an issue of time in punishment, as otherwise it may be asked that why should someone, who in a short moment in their life did something awful, like killing a child, for example, face a lifetime in prison.

As such, one responses is that the time in which humans are tested within a corpreal plane is enough for them to make a decision about their actions. There are only certain people that will face everlasting punishment and they are the type of people that are not repentant for their wrongful actions and as such would continue performing them into perpetuity. They are therefore deserving of everlasting punishment as given the opportunity they would oppress in such a manner too. Everlasting punishment is, therefore, the correct place for them. 

May you always be successful