www programming

Yes, that's exactly that.

But whaterver if i am on WP-Side, or phpBB Side, i'm disconnected for the both when it happens.

hello, so you result logged in, after you've check remember me login into phpBB and executed the login.
Now, this mean all is working about cookies, that are setup by phpBB as a remember me login that will persist for long time (as long, as you have setup it in phpBB i assume).
With login done into phpBB full forum page, and coming into WP, now when the session expire you're logged out?
You should not, or something into WP side fire the logout code somewhere (that is very strange), or your phpBB maybe have not correctly setup the value into Security Settings -> "Remember Me" login key expiration length (in days):
set to 0 if it is not.
This is very strange that session expire if you are logged in correctly and so you have a valid cookie that each time page load, renew session if expired. Let know.

Sorry, i didn't understood the question, but now i do.
I made the test : i set 60s (the minimum) and i've not been disconnected.

there is no setting about this into admin WP config, these are advanced options that need to be setup manually in case into wp-config.php file (i assume).
Sorry but instead of another question ... you didn't answer to my question. What happen if you do the above?
Link to your site?

For phpBB, all the settings you tell me were already there.

But for WP, i didn't understand where should i set that ?

setup cookie settings into phpBB config ACP to be:
Cookie domain: .yourdomain (or if localhost live as is)
Cookie path: /

which i think is already into your phpBB.

Allow "Remember Me" logins: set to yes
set to 0 value for "Remember Me" login key expiration length (in days):
into phpBB security config ACP.

Save.

Now logout phpBB. If any changes on cookie settings as above, clean cookies on browser, and re-login as remember me into phpBB.

Now go to WP. You should be logged in, and what happen when session expire in this case (login done phpBB side full forum)

to test this about session, setup session length to a lower value like 10 (10 seconds in not on online site) so you can see if when after login, and after 10 seconds, your user is logged out or, as should not is logged out because, remeber me login, if a correct cookie is released, should not let expire your session. This is the setting into
Server configuration -> Load settings -> Session length

what happen?