Starting around ~ 800 or so --> hazy with hints of mirror, depending on the steel type.
Around ~ 1000 - 1200 - 1500 --> imperfect mirror with some still-visible scratches.
2000 or above --> mirror becomes much brighter and scratches are less obvious unless viewed under bright light and/or with magnification.
Some highly wear-resistant steels like D2, S30V, etc., will resist the high-mirror polishing using SiC wet/dry sandpaper alone, past ~1000-grit or so. With those, diamond compounds at 3-micron & finer will work much, much better (and much faster) when used on hard, smooth strops of wood (balsa, basswood, etc.).
Less wear-resistant steels will take a high polish easily with aluminum oxide-based polishing compounds like stick/crayon-type 'white rouge' (or grey). Some of those are aggressive enough as polishers, that sanding up through ~2000 may not be necessary. A dense loading of compound on something like hard-backed denim can be very aggressive at polishing right through coarser scratches, with steels like 420HC, 440A, etc.
