There's a ton of variation here. Here's a general guideline:
For reading passages, you can take one of three broad approaches:
Skim approach: skim the passage in 90 seconds, then take 6-8 minutes on the questions, taking your time and looking up things carefully
Balanced approach: read carefully but briskly over 3-4 minutes making use of the highlighter and then spend ~5 minutes on the questions, only occasionally looking up stuff
Slow read approach: read slowly and carefully over 5-6 minutes, use the scratch paper and highlighter as needed, and then fly through the questions and never look up anything
There's no "right" method here. You have to try things out and find what works for you.
For science passages, there's even more variation.
Some science passages are ludicrously short and easy. Like ~150 words of text and one or two pictures. You can get through a passage like that in a minute.
Other science passages are more like verbal with ~500 words plus several figures plus they describe a multi-step experimental procedure. A passage like that you've really got to slow down, take your time and analyze carefully before rushing to the questions. You could spend up to 5-6 minutes just analyzing the passage itself.
Similarly, a time-per-question guideline is nearly useless in practice because some questions are simple recall and can be answered in 20 seconds and others are involved multi-step calculations.
Don't worry about parsing your time out that finely. Instead, adopt a general guideline:
17 minutes for 2 passages in verbal
16 minutes for 2 passages in science
Check your time every 2 passages. If you're keeping up with those general guidelines, you're in good shape.
