¿Qué significa I can go home at a decent time. ?

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Decent means reasonable in this case. When placed with time--that is "decent time"--a decent time usually means the earliest (or maybe convenient) time or the time you are willing to stay until.

Getting home at a decent (or reasonable) time.
I'd like to leave at a decent time.
From now on, I want to go to bed at a decent time.

For example, for you, going home at 8 PM is going home at a decent time, as opposed to going home at 10 PM, which is not. In that case, going home at 10 PM is something that you wouldn't like to do.

For some people going to bed at a decent time is important because they want to get a full night's rest. A decent time for them could also be a range of times and not a specific time. Let's say anywhere between 7-9 PM is a decent time for them to go to bed. Any time after 9 PM is just an inconvenience, and therefore, not a decent time.