With proper maintenance, it will last as long as you take care of it. A custom loop introduces air and bacteria as well as mixing metals to the system so you must take care of it. This comes in the form of generally emptying and refilling the loop every 6-12 months. A silver kill coil is your best friend and the most simple way to cool a custom loop is distilled water with a silver kill coil. It will stay fresh and clear for a long time. The only issues occur when you use cheap coolant or ones with nano fluids like pastel colors and leave them too long. The particles break down and clog your blocks which is not easy to clean. If you are worried, simply use distilled water and a kill coil with colored tubing to get a look you desire. If you wanted to go soft tubing, I would do primochill advanced LRT series as it doesn't leech plastic particles into the loop as well.