“So much for leaving at a decent time...”
You often have to work late hours. Today you wanted to leave work at 6:00, but you were too busy. Now it's 8:00, and you're still working. You think this to yourself.
So much for leaving at a decent time...
"So much for ___" is something that you say when you're giving up. You've realized that you won't be able to do what you wanted to do. The most common use of "So much for ___" is this:
So much for that idea!
You say "So much for that idea" when you're trying to solve a problem, and you've tried one solution to the problem but it didn't work.
The other way that "So much for ___" works is by adding a gerund afterward:
It's already ten thirty? So much for getting an early start!
Oh no! He saw us? So much for keeping it a surprise.
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":
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".