When checking the error log for this forum I occasionally noticed that one of the errors Google was looking for was an incorrect url. Today I decided to investigate and found that a member had written this incorrect url in a post 4 years ago and a day later another member had spotted the error and it was corrected at that point. During that day 4 years ago Google picked up this post and has kept looking for that non-existent url ever since.

Google also looks for files from another site of mine that only existed online for a few hours 10 years ago when Google was still in beta - and few had even heard of the search engine.

Google never forgets by the looks of things and will keep on returning to find urls it once new about - forever.

Well that answers the question about how long to keep a 301 redirect active.