That message is not coming from CC. It seems to be coming from some web server that is not a calibre web server, perhaps something like COPS. Thirty seconds is a standard maximum amount of run time for PHP scripts. You need to change it in your php.ini.
IMPORTANT NOTE: CC works only with a real calibre content server, not with a replacement like COPS. Even if you make the timeouts go away by editing php.ini, you will not be able to download books from that web server into CC.
EDIT: Also note that the calibre content server must be version 0.9.2 or later, preferably 2.4 or later. I mention this because I see on another thread that you are using a raspberry pi server. IIRC the package available for raspberry pi is ancient, somewhere arount 0.8. Of course I might not be remembering correctly.