I’m a junior, and I got the SAT I out of the way in October (2290), but now I need to prepare for the SAT II’s.
I’m thinking about doing Math II and possibly Chem or Physics in May. If I don’t do so well, I can also take them in June or October.
How hard do I need to work to get 800s on both tests?
Here is some practice test questions. See how you do them.
https://sat.collegeboard.org/practice/sat-subject-test-preparation/practice-test-section-start?practiceTestSectionIDKey=Subject.MATH_LEVEL_2&pageId=practiceSubjectTestMathLevel2&header=Mathematics%20Level%202&subHeader=SAT%20Subject%20Test%20Practice&conversationId=ConversationStateUID_2
Here are official study guides.
https://store.collegeboard.org/sto/productdetail.do?Itemkey=009751&track=satsite&affiliateId=satstore&bannerId=subjecttestguide
https://store.collegeboard.org/sto/productdetail.do?Itemkey=007727&track=relatedproduct&parentsku=009751
@payn4ward Thank you!
Depends. How much physics/chemistry experience do you have? SAT II Physics is nice in the sense that the curve is fairly lenient - but this is because the test covers a lot of different content (kinematics, gravitation, rotation, electricity, magnetism, optics, waves, thermodynamics, etc…) so you need to be familiar with a wide range of topics to comfortably get an 800. The content covered on the chemistry exam isn’t as broad, but there’s also less room for error - I think that missing more than 3 questions will get you less than 800.
I agree with @payn4ward - try some official practice questions and tests and see which test seems more accessible to you. If you’re really not sure about which test you want to take you can always try both 
Personally, I managed to get an 800 on both Physics and Chemistry after about a month of reviewing using a combination of Princeton Review and the Sparknotes website - but I’m also in my third year of chemistry and second year of physics, so my preparation consisted more of reviewing and less of actually learning new content.
you should definitely be studying for the math 2. the material is not that difficult and the curve is pretty lenient (I think its 6 or 7) but the time pressure is immense. definitely be comfortable with skipping certain questions (for example I skipped all the probability questions) and using your calculator with ease.