I put relaxer on my daughters hair last night, and being the first time we were cautious and only left it?

in the minimum amount of time and the back of here hair did not relax at all, how long do i need to wait before applying the relaxer again to the part that didn't previously relax? she is biracial her hair is fine but extremely curly and its always painful when i brush her hair no matter what products we use to ease the tangled mess!!