I'm not really sure aout those authors. Both of them were early to mid 20th century writers. I also read to my fancy. I read classics and anything new that I can get my hands on. I believe the time is determined by generations because the world changes so much from a parent's youth to their kid's youth. I don't believe there is a definite cut-off point, but a work must be held in high esteem for at least a few different generations. For a work to be popular in multiple generations, it must address the human condition- something that time doesn't change. This is why great works apply to all generations, whereas, mediocre books may only be interesting to a certain age group. Does that answer your question? I can give examples if I'm not being completely clear.