Some of my husband's family is NO, and we've discussed this in the past. Please keep in mind that this is just what these individuals told me; I have no first hand knowledge about this.
According to them, "retired" NO priests do not enter anything like lay retirement, at least for secular NO clergy. If they are still physically and mentally able, retired NO priests are usually attached to a large parish somewhere, but with no official role. While there, they administer the NO sacraments as they are able, and under the direction of the NO pastor, but have no administrative work. For example, at the NO parish of one my in-laws, there is a retired priest in his 90s who regularly hears confessions, but rarely says public Mass because he cannot stand for long periods of time.
Whether this is typical for retired NO priests across the country or the world, I have no idea.