Users keep having to login?

How do you do it, so that users on a PHPBB3 forum are just always kept logged in.

Maybe your cookie settings?
Here’s from the faq…

If you (or your users) are, after attempting a login, being returned to the index (or other page) without appearing to be logged in the most likely problem is incorrect cookie settings. phpBB uses cookies to store a session id and a small amount of user data. For this data to be stored correctly the cookie domain, name, path and secure settings must be correct. You can check this in Admin-
>General->Server Configuration-
>Cookie Settings. Typically the cookie domain can be left blank and the cookie path set to / (a single forward slash). Do not set the cookie as being secure unless your board is running over a secure sockets layer connection, ie. https://
If you still have problems try setting the cookie domain to your full domain name, e.g. www.mysystem.tld ,
www.something.mydomain.tld . You must ensure the domain name contains at least two dots or browsers will be unlikely to recognise the cookie, e.g. .mydomain.com ,
mydomain.com . Do not add http:// or anything else to the domain name!

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Thanks for that. Did go through that faq and applied it where relevant, but still, when users login, they get logged out within a few minutes.

Just a thought, but when you purge the cache in the ACP, does that affect whether users should need to login again?