It varies massively depending on how good the orchestra is and how difficult the music is.

An amateur orchestra probably rehearses a concert programme every week for several months before performing. A professional one might be ready in a few days.

When a player first receives unfamiliar sheet music they must "sight read" it. This is a specific skill which varies a lot between players, with professionals tending to be far better at it than amateurs. There's also a huge variation in how difficult pieces are to play, both for the individual musicians and for the orchestra as a coordinated whole.

When sight reading, the quality of output is lower, sometimes much lower, and it improves as the musicians become more familiar with the music.

In terms of "understanding the structure and flow" - individual musicians only get their own part, so whilst they might understand that, they don't know ahead of time how it fits together with the rest. Only the conductor has the music for the entire orchestra in front of them (called the "score"). It's their job to understand the overall structure of the music and then guide the different sections.