This might be by design for 2.5.x and 2.6.x, but I do not see it in current release notes for 2.5.3, so at least that documentation needs to get addressed.

Our 2.5.3 tester reports:

Version 2.4.6 does not close tailable cursors after a timeout period. This is why we’ve been able to leave the app up and running for days/weeks with a cursor blocked waiting for events. I verified this today on my Windows instance and on a linux instance.

Version 2.5.3 closes tailable cursors after 10 minutes. The client throws an exception MongoException.CursorNotFound. I have modified our code to rebuild the cursor if this occurs.