How long before my mother fully recovers from mitral valve repair/replacement?

One to three month. Mitral valve repair or replacement is an open heart surgery -- and the recovery process will include a cardiac rehabilitation that can take up to three months for full recovery and functionality -.
Variable. A full sternotomy procedure can take several months for the patient to feel 100 %.
Usually 3 months. I typically tell my patients recovery from heart surgery is about 3 months. That is how long I like to give the breast bone to heal before I let then resume normal activities w/o restrictions. Most feel pretty good after 4-6 weeks. A lot depends on how sick they were before surgery. Also obesity, lung disease, and other comorbid conditions can change this.