Well lets see. Mags shield has easily resisted shots from Thors hammer and She-Hulk combined. Mags has also shown that he can easily use his magnetic power to free himself from Hercules grip.
From my understanding the current Thor seems to be capable of giving WWH some trouble so could we assume that if Mags can take shots from Mjolinor and She-HUlk that the shield could last a long time?
I think if we look at Mags feats I think he should be able to last a long time. WWH had a brief scuffle with Hercules but Hercules wasnt really trying to fight back he would have most likely lost but should have been able to put up a bit of a fight...and if Mags can toss Hercules to one side....
Im giving Mags several hours......becuase his shields durability seems to be comparable to what WWH strength can do ie overpower Hercules with not much effort, but at the end of the day WWH can reach much highier levels of strength more quickly than other versions. Hes been able to take BBs scream I dont know how long he was fighting for but if he can reach that power level during a fight Mags could hold off for quite awhile but not for very long time.