You cannot use average here, since the actual length is affected by how good of a rent deal you have to start with, and in the case, if the tenants start with a

You cannot use average here, since the actual length is affected by how good of a rent deal you have to start with, and in the case, if the tenants start with a good deal (for them), they are less likely to leave after the average period has passed. I don't know if you meant the same thing with the word "endogenous".