I've found that it's less about how long you practice and more about what you practice and how you do it. Just find or create a drill that targets your weakness and drill it slowly paying attention to precision. Whatever you are focused on during the drill (right hand, left hand) is what you are building skill at. Resist the urge to play the drill faster. Every time you flub the drill you are destroying what you been building by playing slowly.
There are no shortcuts. Don't expect instant results, but you will notice improvement fairly soon, within a few days probably.