The answer to your question: Depends on how you've set up your routine, and depends on how fit you are. The more fit, the more intensely you're going to be able to train.

Personally, I workout most body parts more than once a week, and I divided it up so that I'd get 4 days of rest after one workout and 3 days after the other. I go to failure on the day when I'll have 4 days before my next workout, and I leave my second workout for a less intense, higher rep day where I'll focus on whatever body part I didn't hit really well in the workout 4 days before it.

In general though, you don't want to ALWAYS train to failure as that might be a little too much on your body. Even on your heavy workout days , you want to make sure to pace yourself a little bit so that you have enough steam for your whole workout. This doesn't mean that you train half-assed for the first half half-hour in the gym. What it means is that you do the proper warm up for the first few sets, and then go to the heavies weight you're going to lift that day and go to "near-failure," maybe leave with a rep in you for your last few sets.

That's what I used to do and I had packed on a shit load of muscle.