Does oxidation inhibit caramelization?
If the plantains were on the greener side, they would take longer to fry than riper plantains like you have.  The more ripe plantains are, the higher their sugar content, and the faster they will caramelize.  The only effect of oxidation is an increased speed of caramelization.  They will be done when they are soft and brown, but be careful not to let them burn since the sugar content will make them brown in less than 10 minutes.