The singer has been in rehab since being hospitalised for an apparent drug overdose in July.
"She has 90 days," Garza said in an interview on a radio show, according to The Hollywood Reporter.
"I couldn't be more thankful or more proud of her because addiction being a disease, it's work. It's very hard. It's not easy, and there are no shortcuts," she said.
Garza said she isn't sure what led to her daughter's relapse.
"I knew that she wasn't sober," she said of Lovato, who apologized for no longer being sober in her ballad "Sober", released in June.
"I didn't know what she was doing because she doesn't live with me and she's 26."
She recounted the "nightmare" she went through when she heard about Lovato's "overdose" via a text message.
In August, Lovato had shared an emotional message on social media about overcoming addiction. "I now need time to heal and focus on my sobriety and road to recovery," she wrote on Instagram.
"The love you have all shown me will never be forgotten and I look forward to the day where I can say I came out on the other side. I will keep fighting."