Is there a max to the session expiration time which can be set in the admin?

Some side info: I use the autologin hack. The reason I want to extend the session expiration time is to give visitors more time to write their contributions. Right now people sometimes see their work deleted because at the moment they press the post button they are not logged in anymore. The autologin hack does not help with this.

I have extended the session expiration time but it does not have any effect.

I read the FAQ on controlling login session time. This raises the question: how many users can be online concurrently?

I set "Use custom session" to Yes.

In the admin settings it tells me that the session expiration time can only be altered if php_native is off, but where can I find php native in order to check this?

I searched the site but didn't find answers to these questions.