During working hours it is really quiet, also Friday nights are pretty deserted.
The best time to go though is whenever you have time. Don't put off the gym because it will be busy. Doing some exercise will make you feel a lot better than doing no exercise. Unless you're training to a strict regime, where you need to do specific exercises just make sure you put in the effort.
Outside of what I've already mentioned, early mornings are usually better than after work. 630-8 my gym is pretty decent.