For something like Civ or Stellaris, I'd count "completion" once I've won at least one game. Because, ideally, I've shown some mastery of knowledge, skills, and mechanics that allowed me to win. I don't need to play and win as each leader in Civ or every race/trait and combo in Stellaris to say I've completed it.

This is similar to how I'd view "completion" in open-ended games like Cities:Skylines or Banished. Having played a city or town for several hours, was I able to keep the residents alive, stabilize the city if there were any issues, and also grow and develop the settlement for a significant, though arbitrary, length of in-game time? If the answer is Yes to all of these, then I've "completed" the game. I've understood how things work in the game. Doesn't mean I have to understand every nuance or know every little trick. But I know enough that things are going well and largely continue to go well. And every time I start a new map, things tend to always go well.

Earlier this year, I stopped playing Eve Online for the nth time after mostly playing straight through since 2019. Because I viewed my time during this last 4-5yr stint as "complete." I achieved practically all the goals I set out to do: join a major alliance, join massive PVP fights, engage in smaller PVP fights, make money that I ever had before, buy and fly ships I'd never used before, learn how to explore and navigate wormholes, try out specific types of industry, play with IRL friends, own and run my/our own station, and more.

In all of these, "completion" obviously doesn't mean I'll never go back. There's always more to do, new things to see. But for now, I am satisfied with my progress, experience, and understanding. I'm no longer a noob.