Wow, 18GB! It's a huge thing.
Some webhosts do mention on their terms, FAQ or Acceptable Usage Policy. Most of them will limit your max process time. For example, this is mine:
We expect all dynamic (PHP, etc) scripts on our servers to run for no more than a maximum of 60 seconds per process. If you are connecting to external sources (such as Twitter), please ensure that you set the timeouts low enough accordingly to not breach this rule. Scripts running for longer than 60 seconds will be automatically terminated. Repeated terminations of scripts running for longer than 60 seconds may result in suspension or termination of your account. If there are scripts that absolutely must run for longer than 60 seconds, and are critical your business, please get in touch with our support team to discuss running these.
If you need too backup large files, I suggest you contact your webhost first.