Lignum-Vitae Bearings and how they last.

I have no way of monitoring the bearing it is about 12 feet under water I do drain the pit to do maintenance ever 3 to 4 months the bearings have around 8,000 hours on them and believe or not it will wear the shaft more than the bearing. the shaft has a bearing sleeve on it that is harden to around 55 Rockwell and when I rebuilt them they had around 6,000 hours on them the bearing had around .003 wear and the sleeve had around .012 wear.