So far I couldn't find anything really, but is it true that curl doesn't really time out at all?
Will curl always wait (or is it depending on configuration) or is there any sort of timeout?
curl has two options: --connect-timeout and --max-time. Quoting from the manpage:
Here (on Debian) it stops trying to connect after 2 minutes, regardless of the time specified with --connect-timeout and although the default connect timeout value seems to be 5 minutes according to the DEFAULT_CONNECT_TIMEOUT macro in lib/connect.h. A default value for --max-time doesn't seem to exist, making curl wait forever for a response if the initial connect succeeds.
What to use?
You are probably interested in the latter option, --max-time. For your case set it to 900 (15 minutes).
Specifying option --connect-timeout to something like 60 (one minute) might also be a good idea. Otherwise curl will try to connect again and again, apparently using some backoff algorithm.