It varies by country. You should apply for a visa as far in advance as possible but no less than three weeks before traveling in normal travel periods. During peak travel periods (from May to August and around the holiday season), you should apply at least six weeks in advance.
Student visas are usually processed faster than visitor visas and there are specific instances where you can apply for emergency visas, which take less time to be processed.
If you need an emergency visa, click here for more info on how to get one.
Click here to check the waiting and processing times for visa applications in your country.