Czech fluency expressed as a percentage...
One of the things that has kept me motivated with learning German with Duolingo is that it gives you your percentage fluency. So far, there is not percentage fluency in the Czech beta version. Will it be incorporated later?
This is an overview of how percentages are defined in Duolingo: https://englishtest.duolingo.com/scores or at least, how they are defined in their official English fluency test. "Expert" probably means "near native": speaks the language like a native speaker. Very few people get there other than living in the country for a number of years. "Proficiency" is what your target should probably be: be able to speak and read the language fluently in all situations, while still occasionally needing a dictionary and still having an accent.
How would you define a fluency percentage? What does "100% fluent" mean? Or "80% fluent"? My estimate is that after you finish a tree on duolingo, and you repeated the materials a number of times, you know just enough to start really learning the language. Whatever percentage that is. Don't bother percentages or stages or numbers. Enjoy learning Český, and after completing the tree, buy novels in Čech, go to Prague for a couple of weeks, and keep learning.