You have to find your own limit and really examine your goals. Training to failure every single day will eventually lead to burnout and reduced training effect, but planned over-reaching is the only way to continually improve. You have to find the point where you make improvements without overdoing it, and be sure to maximize your recovery in every way possible, through proper sleep, proper diet, and any recovery enhancement (such as contrast hydrotherapy) that you can manage. If you eventually start to feel a bit beat up, back off a bit, take some planned rest, and come back ready to push your limits again once you're fresh!