“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 to yourself:

So much for leaving at a decent time...

So much for (doing something)

"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!

"So much for that idea" is something that you say when you're trying to solve a problem, and you try one solution that doesn'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.

a decent (something)

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".