It is hard to say anything on this matter and not introduce much opinion; that being said, I will try to make this as non-biased and founded in fact as possible. Moreover, I will try to make this general to fit a large-audience and to be applicable to many types of textbooks.
The first thing you need to consider is the style of writing. I have read texts in which the examples were critical because some of the key information is only found in the examples. On the other hand, I have read texts in which there were almost no examples at all, either because the author found them unnecessary or because many examples exercises were already worked throughout the chapter, or perhaps both. I have often found that the author will make note of this early on, perhaps in a preface — to point out the obvious, if you don't understand how the author intends you to work through the textbook, ensure you didn't skip over instructions from the author.
The second thing you must consider is your own knowledge of the subject. I can sit down and read through many real analysis texts and understand everything without working any examples, but I cannot yet do this when reading in-depth analyses on other subjects such as Contour Integration or Fourier Series. Taylor the number of exercises you do to fit your needs. While the consequences of doing too many examples seem a matter of debate, the consequences of doing too few examples can quickly become apparent when you don't understand material later in the text relating directly to the examples. Lack of understanding is of course not always solved by doing more examples, but going back and working through some problems you skipped can help you figure out what critical details you missed.
A third factor is time. Life gets stressful and busy at times, and you might naturally want to do less examples during these periods to be able to cover more topics. On the other hand, you might want to do more exercises during these times to keep yourself from forgetting the material. I have yet to see a universal rule for studying as much as possible in a restricted time frame, though trial-and-error has helped many students I've worked with. If you feel cramped for time and don't feel like the strategy you've been using is working, try a different strategy.
It is probably important here to note that a strategy you have used for one text might not work for another. On top of that, different conditions in life might necessitate different reading strategies. There are many different learning styles, and not all of them might apply to you. Though I must admit the following line is opinion, I feel that I must include it: If you feel like you aren't learning as well as you could be, try something else. If you feel like your learning improves after the change, great! If not, then either keep trying new techniques or try to adjust other factors in your life that might be influencing your ability to study most effectively. Of course, it would be a folly to devote all your study time to optimizing and not to studying; I shamelessy reference XKCD:
Note that this flowchart could be considered opinion and will not guarantee success in all you aspirations.
