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.

It keeps going and going and going....
LOL We should make a google commercial.

Yeah the same thing is going on with a few of my sites. I built the sites one way when I first got them. Then, I learned about SEO and totally redid my sites and google is still looking for some sub pages and sub URLs that don't exist anymore.

You don't need to go further than this: http://www.alexa.com/siteinfo/agentsonline.net So, for this very site 302 error is reported for ALEXA!

I was under the impression that if a url returned a 404 error then eventually Google would stop looking for it. But in every thread I have posted on I have had to qualify my comment because the OP was a long time ago and things may have changed.