An income tax return has to be filed by May 31st (for 2018: July 31st) for the previous year. It is possible to file for extensions. If you work with an accountant you will automatically get an extension until the end of December (for 2018: until the end of February of the following year).
If you are a single individual (or married couple with one salary only) who has nothing but income from work as an employee and who has worked in Germany for the full year, you have no obligation to file a German tax return at all (note: this can be different for married couples who are both working). However, you can voluntarily file a tax return if you expect a tax refund based on your work related expenses. This possibility is available for the last 4 years – sometimes even more.