You can come at this from multiple directions.
On the one hand, it’s wise to only expend effort making something faster up to a point. At some point, unless a human has to wait for the result, there is no reason to make something faster [1].
On the other hand, once something takes more than a minute or two, and the person who started it goes and does something else, it doesn’t matter how long it takes, as long as it’s done before you get back. Film shots usually render overnight, so as long as they’re done in the morning and as long as they don’t prevent something else from being rendered by the morning, it doesn’t necessarily need to go faster. Somewhere out there is a blog post I remember about writing renderers and how artists behave; it posits perhaps there’s a couple of thresholds. If something takes longer than ten seconds to render, they’re going to leave to get coffee. If something takes longer than ten minutes to render, they’re going to start it at night and check on it in the morning.
[1] I always like the way Michael Abrash framed it:
“Understanding High Performance: Before we can create high-performance code, we must understand what high performance is. The objective (not always attained) in creating high- performance software is to make the software able to carry out its appointed tasks so rapidly that it responds instantaneously, as far as the user is concerned. In other words, high-performance code should ideally run so fast that any further improvement in the code would be pointless.
“Notice that the above definition most emphatically does not say anything about making the software as fast as possible. It also does not say anything about using assembly language, or an optimizing compiler, or, for that matter, a compiler at all. It also doesn’t say anything about how the code was designed and written. What it does say is that high-performance code shouldn’t get in the user’s way—and that’s all.” (From the “Graphics Programming Black Book”)