Given the circumstances, not the best comparison you've made Skillz.
Vader was tanking Marek's lightning, lightning powerful enough to fully empower a weapon that blew clean through a Star Destroyer. Vader takes Starkiller's full lightning storm amplified by two pylons and doesn't even appear injured afterwards, simply subdued. ROTJ Vader > TFU II Vader.
Given Sam Witwer's commentary, Vader was half-hearted the whole time and wasn't going all out. I'd wager he's going to be able to hold on quite some time, especially given his monstrous durability and endurance.