How long does it take for a new podcast episode take to update?

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.

You should check your feed by subscribing, either from the Store page or manually from the 'Advanced' menu in iTunes, to see that your new episodes are there. If not, then you have an error of some sort (such as a duplicated 'guid' tag). If all your episodes appear when subscribing then the Store should catch up after a bit.

As you've not posted the URL of your feed and that of your iTunes Store page I can only give you the above general answer. There may be other factors, but I can't say if this is the case without being able to look at the evidence.