I've made a 6k resolution render with just 8GB RAM once. I don't remember the rendertime, but it was under a day, with quite high quality settings and a relatively complex scene.
Generally it's impossible to say how long a render takes - it all depends on the complexity of the scene and a lot on the chosen quality settings.
With very large images, that are meant for print, you don't need to use very high quality settings, as the viewer will probably stand a meter or more away from the image.