What happens in glacier after you initiate an archive retrieval job

I'm writing some code to retrieve archives from glacier. My code initiates a job then listens on the queue for a message with that jobId, then does the download of the archive. Documentation says that the jobId will not expire for 24 hours. Does this mean that if you don't complete the download within those 24 hours you can no longer download and need to initiate a new job? And does the 24 hour clock start when the job starts or when the job finishes?