If you still have control and don't have a burning need for it otherwise - I would plan to leave the 301s in place permanently. If you want to use it for something else.. well, really a year should be enough.
But, as with all page changes - try a few pages, and see what happens.
In particular, you should keep 301s permanently in place for any old urls that still have inbound links pointing to them.
Google does not retain the redirect information for further spidering. If you remove 301s to old urls with existing inbounds, Googlebot will fail to reach the new urls, and you'll lose linking credit for those links.
Chances are that while you may have gotten some new inbounds in the year since you made your changes, you still have a lot of old inbound links out there that haven't been changed.
I generally tell clients with established sites to expect to keep the old domain in "perpetuity".