Until now all my hones have been well used and lapping them was more of a cleaning effort rather than making sure they were flat.

Now that I've got a couple brand new ones that I've lapped flat (by drawing the pencil grid, lapping, repeating the grid and lapping again just to make sure) I'm wondering how often I should do the grid thing? Is this a once in the beginning sort of thing or do most folks repeat it...and if so how often?