In the same tax year, the teacher must stay more then 183 days, then he/she can try to file the tax refund.
The first 183 days, it’ll be 18% of your salary, after 183 days, it’ll be 5-6% of your salary.
e.g. if you come in September, then from Sep-Dec, your salary will be deducted 18% ; if you come before July, then you will still be deducted 18% between July-Dec but can always get it refunded the next year.
Even if you’re not in Taiwan that time, you can still have it done on line.