The word "decent" means "good enough". For example, if a restaurant is not too bad you can say that it's decent.

The word "decent" can also be used in the phrase "a decent chance":

I think I may have a decent chance of winning it.

English speakers often use "decent" to talk about numbers and time:

If you went to bed at a decent hour, you wouldn't be so tired.

I hope we can get a decent number of people to show up for it. (Talking about a party)

Leaving "at a decent time" means not leaving work too late (or too early). If the official end of the work day is 5:00, then leaving by 5:00-6:30 or so is "leaving at a decent time".