Sick leave for extended period of time

Hello, I have a question regarding a sick leave and employee’s eligibility and the same employer’s rights. How long does an employer have to keep a job open for an employee on sick leave? The Government of Canada Website indicates that an employee is protected for any absence not exceeding 17 weeks but what in case the employee exceed the limit, what are the employer’s rights in case when employee delivers doctor’s certification for extended period of time, over these 17 weeks. Please let me know, thank you.