Depends on too many factors
First of all the application
Has the applicator of glass coat applied the glass coat the perfect way.
2ndly how are u maintaining the coat
If you are washing with surf and peela kapra. Its life will reduce easily to 1 year
If you are using microfibre and are not dry dusting the car and washing it with shampoo and mainting the glass coat it can last upto the claimed life given it has been applied correctly by the detailer.