How long before should I start studying?

I usually start studying about a week or two before if it's an important exam, but it really depends on how much material there is and how familiar you already are with the topic. If it's stuff you've never seen before, it's better to start earlier - maybe three weeks - so you have time to actually understand the concepts without cramming the night before. The most important thing is not to wait until the last minute because you study poorly and forget everything quickly anyway, it's better to spread your studying over time even if the sessions are shorter.