My mother usually sends packages by express air mail, and we get them within 5 days. However, when she was sending them "plain" air mail, they did take about 1-2 weeks. These would be office paper box sized, and usually pretty heavy.
We always get a slip through the door if we don't happen to be home when they try to deliver. My experience with Parcel Force is that they try twice, then tell you to come collect at the depot.
Also - my mother puts on the form NOT to return the package to the US, but puts my in-laws' address as an alternative UK address if for some reason it cannot be delivered/picked up by us. That way it doesn't bounce back to her.