How often...?

I think part of it depends on how long your hair is. If it's pretty short, most of the damaged parts will be cut off fairly soon, anyway, so you won't have as much to worry about with damage. If it's long, you have to be a lot more careful. If it's in good condition still, you could dye it now, but if you're changing to permanent dyes, it's probably best not to change the shade often, and make sure to use the touchup technique every time so there's less damage accumulating on the length.