As early as realistically possible.
I say this because some people might think it is possible to practice doing MCQs from first year itself but that is simply not useful for a majority (~99.9%!) of the people.
You’ll forget it.
How long do you think you can keep revising something with proper spacing? Through 6 years and 14 subjects?
If you were that efficient, you’d be extraordinarily successful already! Like those geniuses we hear about every now and then who invent things in their teenage.
I hate to break it to you, but if you’re here and asking this question then you’re not a genius. You’re only marginally smarter than me because at least you’re asking this question before it’s too late!
So, coming back to normal human beings, the middle of second year to middle of third year (Part-1) is a really good time to start your preparation. If I had to do it again, this is what I’d have done.