honestly I don't see the point in having over 2k samples
like the only reason i would ever go above that would be if i was making a final render for a paid project but even then 20k seems quite high.
Well, even at 2K if you zoom in there will actually be very slight pixel hue/color variation, So if you need absolutely pure colors then the only reasonable limit is the time its gonna take to render.
20K for 5 hours is actually pretty good, considering the sheer quality and perfection of the render.
I mean, you could just leave it on while you sleep, or when you go to school (what I personally did) and It's a final render so you won't (hopefully) be re-rendering it.
5 hours is pretty measly when you think about it.