This depends on the amount of time and dedication you put into it plus your natural ability to understand and represent the world visually. Which isn't the same for everyone. Nature and nurture both make a difference.
In short I've seen most people make significant progress in the first year or two of their art journey, but after that you sort of hit a plateau. Here's where the difficult stuff begins and it's a life journey. Whether or not that is good enough for you and what you want to draw is only for you to decide.
Okay, I don't know if the word has a very specific meaning in contemporary military vocabulary.
All I know is that a mercenary is an umbrella term for a "warrior for hire". It's typically associated with having to do jobs most others don't want to do, either because they're very demanding physically or because they're morally questionable. And this is where your confusion with assasins and hitmans comes in. A mercenary may also be a hitman, it depends on the context. But not necessarily. Usually mercenaries are supported by ( because they work for) either the government or some other legal entity. An assassin or hitman usually doesn't, or at least isn't supposed to.
In other words, a mercenary has a reputation for taking morally questionable jobs but isn't regarded as a criminal, whereas assassins, hitmen, and evil henchmen are criminals from the get go.
I hope that helps