I went back to work in November, and have started riding (with a courier bag) daily last week.
I was told that the bone should be as strong as the other within a week of the surgery, but it was still very uncomfortable trying to carry a bag across it.
Take it easy now, and get the proper advice, you'll know when it feels right to use it in anger.
Edit: TomCrow, I had similar feelings with the plate in, wearing a rucksack would trap the skin betwen it and the plate.
The plate is only there to provide scaffolding to allow the bone to reconnect and heal. Once that's happened it's surplus to requirements, as the healed break will actually be stronger than the rest of the bone.