I have 2 accounts on iso, one with a few thousand games, one with a few hundred (I was curious to see what would happen if I made a new account where I didnt play 1/3 of my games against my wife)
I after a couple months I ended up literally tied with myself on the leader board.
Despite being in the top 10 at one point I am a level 40 player. I don't have the patience or discipline to win at the rate required of a level 50.
Here is how I'd rank players:
Level 0: Who knows, maybe a noob, maybe a new account.
Level 10: Usually pretty bad or new to the game/playing online. Even people who don't play very regularly will get to 20. Usually doesn't know all the cards or the combos. Will frequently ignore powerful card/combos and make rookie mistakes like buying lots of terminal actions.
Level 20: Knows all the cards, has a good concept of common combos. Can read a board and identify most strategies. Regularly makes mistakes in missing an important card, or will "try out" things that sound like they might work but are not very good in practice.
Level 30: Experienced ISO players, knows every card, good at picking out the likely "best" strategy and sticking to it. Not as good at adapting, or making strategic decisions based on the current point score/deck situation. Will sometimes miss great combinations like double tactitian or hamlet/watchtower.
Level 40: very experienced players, good at advanced tactics, making strategic decisions on when to shuffle, when to buy penultimate province, does card counting of both his and his opponents decks to aid in making best decisions based on probability. Most common mistakes here are Fancy play syndrome, where they avoid simple strategies like double Jack or IGG-rush in favor of something more elegant.
Level 50: At this level you often have chessmasters playing dice. You'll see players playing coppers one at a time to hide their hands. In game reports you'll occasionally see "mistakes" but rarely were those choices not intentional. Just choices made that didn't work out because of luck/bad shuffles.