Spotting a rare creature

This week’s Riddler Classic is a question about large numbers of attempts at a very unlikely thing.

Graydon is about to depart on a boating expedition that seeks to catch footage of the rare aquatic creature, F. Riddlerius. Every day he is away, he will send a hand-written letter to his new best friend, David Hacker. But if Graydon still has not spotted the creature after $n$ days (where $n$ is some very, very large number), he will return home.

Knowing the value of $n$, Graydon confides to David there is only a 50 percent chance of the expedition ending in success before the $n$ days have passed. But as soon as any footage is collected, he will immediately return home (after sending a letter that day, of course).

On average, for what fraction of the $n$ days should David expect to receive a letter?

My solution:
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