My mom had undergone a dentigral cyst (cyst under the tooth) surgery. from than onwards, her mouth is not opening to full extent. since, it is now one month or so after surgery,is it normal? how long will it take for her mouth to open completely ?

What's the symptom prevent her from fully opening her mouth? The healing time could be just a week to months depending on the how extensive the procedure. Did she need the bone graft? She may need a second opinion if her oral surgeon or dentist does not provide a satisfied answer.

Not sure what a dentigeral cyst is. Do you mean dentigerous by any chance?

I don't recognize "dentigral." That aside, communicate your concerns to the General Dentist or specialist Dentist that treated her. There are lots of reasons for her altered function, and she will have stop be reexamined to begin the diagnostic process.