as Aftermathman said. When a meeting is arranged with the head of human resources with the company, they usually just ask how long you think you may be able to work. In my case they continued to observe my performance and when it appeared that they were flogging a dead horse, they invited me to go on STD, and i went directly on LTD because they knew that there was "no getting better" with PD. After that SSDI put me on the roles with a simple phonecall and a check n with my neuro.
It is obvious when "we are done". My immediate boss used to encourage me years efore i went out to stop working , so I could "have a few good years to travel around and have a retirement" because he was aware of the long cold winter that was about to set in.
BUt like a good drone, I hauled my butt in every day until I told them "i want to come in, but the left side of my body won't bring the right side in". THis is a reference to the fact that some PD starts very severely on one side of the body, before both sides are equally (or not) affected.