How long should I keep 301 redirects?

[socialtowards]

I have modified a the URL structure of a whole section of a website and used mod_rewrite 301 redirect to match the new structure. Now that was around 3 months ago and I was wondering how long should I keep this redirect for?

As it is a new website I am quite sure that there are no links around with the old URL structure but still I can see the google bot trying from time to time to access the old URL structure. Shouldn't the google bot learn from this 301 redirect and not go anymore for the old URL?

[socialtowards]

Thanks to both of you for your answers. I will then leave my rewrite rules just in case. I guess they are not slowing down access to the website as they are really simple and quite standard. I suppose that google takes longer to "learn" the new URLs through the redirects.

[HalogenDigital]

I would agree with Marcus' answer on this one.

Another question as an off-shoot from this one would be: What is the performance loss in checking each request against your htaccess rules?

If the answer is none, or very little, then perhaps keeping the 301 forever isn't a bad idea at all. You never know how long people will be linking towards those 301'd pages. I personally have some pages just like this which have been redirecting visitors for as many as 5 years, because it's better to keep the link juice than to end up as 404s.

[Marcus_Miller]

The simple answer is... forever, unless you have some pressing reason to remove them. Certainly, I would keep them there till I knew there was no indexation of these other links and no requests for them for a say, 12 month period.

If you have external links pointing to any of these old pages, then again, that is another argument for keeping them in place.

Just leave them there, comment them as legacy in redirection script and forget about them and focus on something else.

Hope this helps.  
Marcus