It differs from country to country.
For the UK, you must take the BMAT/UKCAT for medicine and the UCAS deadline is usually in October (for medicine) i.e. the October when you've started your A2 studies (twelfth grade).
For US, you must complete SAT and other required standardized tests by December i.e. the December of your twelfth grade and applications usually need to be submitted around Jan 1st of that same year (usually for top universities). And for the US and Canada, you need an undergraduate degree before pursuing medicine
For Canada, application deadline is usually around January to March (depending upon the school you're applying to). Canadian Application process is the less hectic one compared to its other peer countries.
The application process for Australia is similar to that of Canada.
Hopefully those were helpful. Feel free to ask any questions you have.
