It is possible to start earning straight away with a new hub on the day that you publish it.
When you first publish a hub, you will notice that for upto a day after publishing, you will get traffic from Hubpages because your article may be being read by other hubbers. (See explore - top right - hubs. It lists the most recent published hubs and other hubbers go here to see what hubs have been written.)
If you have joined the Hubpages Ad program you will earn every time someone views one of your hubs. So from day one, if other hubbers like and read your article, you are earning.
(Google Adsense is different - it pays you everytime someone pays clicks on an advert on your hub, so no guarantee that someone will click on the first day)
After that first day there are various ways you may get traffic:
Internally from Hubpages - via other hubs on the same subject. (This happens if a related hub shows your hub on the related hubs list on the right hand side of the hub page).
Externally - If you have done your SEO homework and manage to get high up on Google's search results you will attract readers from Google.
Then there are other methods that some hubbers use to increase external traffic further, such as back linking.