According to research, many teachers start to experience burnout within their first few years of teaching, with a significant portion feeling burnt out within the first 5 years, though the exact timeframe varies depending on individual factors like workload, school environment, and personal coping mechanisms; however, some teachers may not experience burnout until much later in their careers. 
Key points about teacher burnout:
Early onset:
A large percentage of new teachers report feeling burnt out early in their careers, often during their first few years. 
Factors influencing burnout:
High student-teacher ratios, demanding curriculum, lack of administrative support, and chronic stress can accelerate burnout. 
Signs of burnout:
Exhaustion, emotional detachment from work, cynicism towards students and colleagues, reduced job performance, and physical symptoms like headaches and insomnia.