Albert Barnes Commentary Job 36:11

"If they hearken and serve [him], They shall spend their days in prosperity, And their years in pleasures." — Job 36:11 (ASV)

If they obey and serve him - That is, if, as the result of their afflictions, they repent of their sins, seek his mercy, and serve him in the future, they will continue to prosper. The design of affliction, Elihu says, is not to cut them off, but to bring them to repentance. This sentiment he had advanced and illustrated before in more detail; see the notes at Job 33:23-28.

The object of all this is undoubtedly to assure Job that he should not regard his calamities either as proof that he had never understood religion (as his friends maintained), or that God was severe and did not regard those who loved and obeyed him (as Job had seemed to suppose). But it was also to assure him that there was something in his life and conduct that made discipline necessary, and that if he would repent of that, he would find returning prosperity and end his days in happiness and peace.