Today we've got the following mail (it was not funny to not leave reply email, so I post it here and answer):

Hi MiniBB Team,
I have been using miniBB without too much trouble customizing it so far. It works very well. Good job!
I have been looking at the forum but didn't find any reference to the issue below:
The manual says: "Registered users (and only registered) can edit THEIR messages, but only if they are cookie-logged and
only if the time of the message is not expired"
I get the need for registered and logged, but how do you define the "mssage time expired?" Is it aftr the post is replied
to , after a certain passing time (how long?) or after the user logs off, that it becomes un-editable?
And how can I change this so that it's always editable? I have looked at the code for a long time without finding it.
Many thanks for your help in this matter. If there is a forum post I didn't see, pls point me to it :)
Best regards,
Stephane

So, expiration time is defined in options, $useredit variable. After that it is as always nice to read manual:

$useredit=86400; Parameter defines number of seconds, after which user WILL NOT BE ABLE to edit his message. By default is 1 day (86400 sec.) If 0 or not set, user will be able to edit his post everytime.