I'm more familiar with wear surfaces on the edge of boards that are inserted into a card rack. So, I'm not sure how to relate to your "hard gold on a slip ring pcb that has brushes riding over the conductive surface." Is your application a board that makes contact to a connector arm as it rotates? [Would you post a picture?]
Number of insertions is affected by the contact normal force that varies significantly between suppliers and designs. Other factors that also affect insertions are plating porosity, plating adhesion, particulate contamination, and contact geometry.
IPC-2221 says something like ... For edge connectors and areas not to be soldered: * Nickel - 2.5um [~100uin] min [Class 2 & 3]. * Gold - 0.8um [~30uin] min [Class 2], 1.3um [~50uin] min [Class 3].
In this, I'd guess: * 30 uin gold => 50 to 75 insertions * 50 uin gold => 300 insertions