How long does it take to to get a transmission status back?

Once you have submitted your STP files and the Message Status says QUEUED it can take up to 7 days for the ATO to respond.

If you have completed your STP submission correctly and your message status is Queued there's no need to panic. If it comes back with an error then you just need to check the files, amend the error and send it through again.

Do I need to keep polling to see if the response has been updated? 

You are not required to keep selecting the Poll ATO Response for submission finalisation button as the response notification is sent to the dedicated email address. Once you have received the email notification then you can log in and poll the response, each time you select Poll it uses a little bit of the data so we do not recommend to Poll if you have not received the email as it will be a waste of data.