Probably about an hour and a half, if you're fairly fit and don't stop too much! It's shingle for much of the way, so it's tough going. If the tide is low, it's sometimes easier to walk the shoreline, or the inland side on the edge of the saltmarsh. Still a bloody long trek, though!
As for timing distance, 3 miles an hour is usually comfortable walking on level ground... but then when you add in ascent / descent (not too much of that in Norfolk
), rough terrain, and time for birding, it can drop down to a considerably slower rate!