The RO style is complex and has a high colour count. Such things take a long time, even when working with templates and predefined palettes as the RO artists (plural!) probably are. Even experienced, speedy artists will likely need at least several hours per sprite, plus many more hours for the animations, many of which have to be done essentially from scratch, as moving already-pixelled limbs around can only get you so far.
If the artist also has to design the characters instead of working from a design, expect to add at least a few more hours for that. Design takes experience and time.