What kind of knife, what kind of steel, what are you cutting most of the time? As a very quick rule of thumb, I'd say, Western stainless at around 2K; most powdered steel and AS all-purpose chefs at around 4K; simple carbons anywhere between 4-8K. But as in all things, it depends. For butchery, I tend to finish around 800-2k.
Most people, most of the time finish simple carbons and similar for all around work at around 3-6K. You don't need to go above 3K, unless you want to try it. You might want to if you have simple carbons and want to achieve a glassier cut. Vegetables cut with a fine polish will stay fresh a bit longer. It's nice for presentation cuts, but hardly necessary.