Hi Dipen
The electrostatic cleaning system is safe to use for any length of time.It has no effect on the oil.The process cannot remove any dissolved components of the oil, so additives are not affected.The idea of using oil forever is not entirely wrong, because in a "clean","cool" and "dry" hydraulic system there is no possibility of oil dterioration.
In such cases, 10% addition is sufficient.This is 10% every year, and not just once in a lifetime.This refreshes some additives which deplete by age. The idea is that oxidation is catalysed by metal wear particles in the presence of moisture, and heat.If you remove the causes, like particles by the Electrostatic system, and control your temperature, the oil stays as good as new. Also, the main property of oil, which is viscosity, never changes unless you mix two oils, or oil and water.
Take 10% as the best case, worse cases may require 20%, but if controls are excercised, complete oil changes are never required.
The parameter to check is Total Acid number, and it can be kept under control for years together.