You don't really have to go too nuts with lapping the back, you're
making it flat for the sake of edge geometry. Just an inch or so at the
end should be enough. You'll still be to hitting the bit at the end to
remove the bur every time you sharpen.
FWIW, I did lap mine over most of the back using sandpaper on glass, but
I don't remember how long it took. Of course, compared to how long
you'll be using them, it's not much time.