When you add a new episode to your feed and upload it, it appears immediately for subscribers because their iTunes application reads the feed directly and the Store is not involved.
However because of the huge number of podcasts in the Store (thousands) the Store caches the feed and checks it for updates periodically. For this reason it usually takes 1-2 days for new episodes to appear in the Store: sometimes it can be less, and occasionally the whole process seems to get stuck and it can take several days (as seems to have been happening today).