From my experience it will take a minimum of 3-4 months. Redirecting an entire successful site as you did is not for the faint of heart.

Going back to the old site will in all likelihood only increase the time it takes to get your rankings back. If you are confident that your original reason for moving was sound, then I would advise you stay true to your conviction.

It will probably get worse before it gets better if it has only been a month now. But, assuming you have done it right, it will get better...

I did a 301 to a new domain name about a month ago.

I assume that the site URL structure was kept as it was and you simply copied the site to a new name and from your registrars control panel pointed the old domain name at the new one?

For example when typing exampleold.com/widgets.html it automatically goes to examplenew.com/widgets.html?

If so then you need some more time, I recently did that with a successful site and whilst Google picked up on it very quickly it took a month before some pages started to re-appear, two months for the majority of them and the images began to be replaced in image search and at 3 months the entire original site has been purged of all the old URLs, all the images have been replaced and ranking is back to where it was before, in fact I think the images are even better than before!

I agree completely with willybfriendly on this ... and from experience. We did a significant 301 update last year, changing unfriendly URLS to friendly URLs. Within one week of putting these in place, our rankings for various terms went berserk. I've never seen fluctuations like it. However, around 3 - 4 months later, these fluctuations became less and less and the site is now pretty much back where it was.

The total time taken since the change has been aroudn 6 months. Some terms are no longer has high as they were, but others are performing far better than before. Our overall view on the changes, the fluctuations and what seem to be our new base rankings is that the whole process was worth while.

We had periods when we has thoughts like yours, but are now glad we stuck to our initial reasoning and weathered the storm.

Garya I hope your plan was well thought out before you took an established domain and pointed it to a "New" Domain as this is not something I would ever recommend unless it was an absolute.

New domain name I would assume has no backlinks so I would advise some hard work to get some links is in order.

Can you explain why you did this?
I am not so sure it is a bad idea to cancel the 301.