A hydraulic oil has a number of purposes, it activates the machinery / rams etc, it cools, it cleans and it lubricates. Any oil is made up of certain performance boosting additives which help the fluid carry out these tasks. now to answer how long it will last, the fluid will be exposed to impurities, like dirt and water, these can be removed with good filtration and separation, however the thermal and lubrication aspects will not be affected by filtration. Unless you carry out an analysis of the condition of the fluid then always follow the OEM interval.