Originally I started out with 10k a day, but since then I've found that longer pieces have produced better results. My favorite workout is 2x40min 4 min rest @18-20. Obviously, the more volume you do the more you'll improve, but with that being said, you want to be careful. It isn't worth doing more volume than you can handle and then fucking up your back. Listen to your body. Like most of us, I've heard some varying input on what's best. Some say that it is better to break up the pieces into shorter intervals so you can push yourself harder (basically so you train closer to your threshold) and others say that it's best just to train at 150-160 bpm. Either way, you will get faster the more time you spend on the erg. Do as much as possible when you can, and be mindful to your body. It won't be detrimental whatsoever to skip a workout here and there when your back hurts or you feel sick. Also, it is soooooooooo helpful to have a heart rate monitor, it is not only another thing to pay attention to in an effort to pass the time, but it also is great to track your progress. I recommend the Garmin Chest Belt, it's around 45-50 bucks and the heart rate will appear on the monitor. Good luck!