This hasn't been verified or tested but there are a lot of references to the 90 seconds in:
/inc/classes/BxDolCmts.php

'sec_to_edit' => 90, // number of seconds to allow edit comment after submit, 0 - do not allow edit

in this file there a several for different comment areas for profile comments, shared photo comments, shared music, shared video, classifieds, and blogs for sure.

If you are wanting to increase or decrease the 90 seconds you would want to make sure you increase all areas.

Again see more this hasn't been tested or verified, but it would be one of the first things I would try.