Domestic Adoption - How long does birth mom have to change her mind?

Does it vary by state? We live in Texas now, but are soon moving to Louisiana. The agency we want to use is in Texas, so the adoption would be subject to the laws of Texas, right, regardless of which state we live in? I've heard in TX that the birth mom only has 48 hrs to change her mind, as opposed to 3-6 mos in some other states. Does anyone know if this is true? It would be hard enough to have the birth mom change her mind at all, but after 6 months of being mom to a child I don't think I could bear to lose him/her. We are just starting to look into adoption, so any tips or info would be greatly appreciated.