Two weeks (with treatments) is fine... by: Camille
When it comes to how much time between relaxers there should be it really more a matter of what condition your hair is in than the time.
If you deep condition your hair once with a protein treatment and two times with a deep moisturizing treatment you should be able to correct the under processed perm in 2 weeks with little to no damage.
Keep in mind that when you go over the under processed hair you shouldn't leave it in very long. Just enough to get the previously under relaxed hair straight.
It would be a good idea to pre-treat the hair that's already relaxed straight with some conditioner before you go again to prevent overlapping and over processing.