The Sefarim discuss (see Hobot Halevavot, Michtav MeEliyahu etc.) the idea of being Mishtadel (making an effort) for one's livelihood but believing that the Hishtadlut (effort) has absolutely nothing to do with one's income.
I have heard from Rav Yoel Ashruv that the proper Hishtadlut one should do is as follows, for example: if nature demands that one work 60 hours a week to get a livelihood, he should work 45 or 50 hours a week.
What is the source for the words of Rav Ashruv? I asked my older brother who says he's never heard of such a thing. What do the sources have to say about how much Hishtadlut one should do for livelihood?