How soon can I redye my hair?

It won’t ruin your hair unless you keep doing weekly. Assuming it was a permanent dye, you really should redye it around 6–8 weeks later, but if it’s not the right hue and it’s the first time you’ve done it, it won’t hurt you too badly (at all) to dye it what you want.

It won’t kill your hair, but you may want to use an extra moisturizing conditioner just to keep it from being frizzy.

It should be okay to re-dye it, although it depends a little on the type of dye you’ve used.

Since you’re going from light to dark, and wish to dye it even darker, it shouldn’t be much of a problem as there will be little or no bleach in the dye. If you were going from dark to light this would obviously be significant.

Also as the previous respondents suggest—don’t dye it too often and use a good conditioner.

After taking cosmetology and having every hair color/style under the sun, you are fine ! :)