how long time to keep the 301 ?

You should keep them as long as they are useful. That is entirely up to you and is based on the context of your site and what the redirect is for.

so you can’t give me an order of magnitude, it’s between 10 years and a day?

No, I can’t.

As I said, it’s entirely dependant on your site and the redirect. Keep a redirect for as long as it’s useful. You will know the context of your site and the redirects, and you can use the log information to help you. There is no right or wrong answer.

The problem is that it doesn’t depend on me but on the engines! and I don’t know enough about the behaviour of the engines. Neither do you, obviously!

because when for example they make the 404s happy with all the possible add-to-cart we wonder if it wouldn’t be better to test my site ! or with page numbers ! or /false, etc. they don’t lack imagination to collect the 404s for pleasure.

Ok, maybe pause and take a step back.

I don’t know what engines you are talking about. I don’t know who they are, or if they lack imagination.

I’ll try again.

Some redirects are because content moved. Some redirects are used for tracking, or for short links, or for any number of reasons.

I do not know what your site is or how you use redirects.

So, back to my original statement. Keep a redirect for as long as it is useful on your site. You can use the log information to see if a redirect is still used.

It does not matter if you keep a redirect for longer than is necessary. It does matter if you delete it too early.

I can re-summarize this again if you need me to.

well, I’m deleting the 301 that have been at 0 for several weeks, and maybe also those that are at 1, and also those that have not been followed for several weeks.