I am a math major at a relatively small college with barely any choice of classes to choose from so I have to supplement my studying with a lot of self studying.
I usually have no problem getting through the chapters and understanding "most" of what is going on in the chapters. Take for example, Abstract Algebra by Dummit and Foote. I tried studying from this text during my summer break. I spent the whole summer studying the text by reading through every section from the preliminary section up to approximately section 2.3. That is I spent 4 months and barely got through 2 chapters. In my own defense of how long it took me to get through the 2 chapters or so that I got through, I solved every exercise in the text (including the exercises in the preliminary chapters) up to all the exercises in 2.3.
My question is, how do I know when a problem is worth doing? Often, I would get stuck on a problem for hours and when I figure it out, it turns out to be surprisingly simple. Is there a time limit I should set for myself before moving onto the next exercise? If I do skip an exercise, when should I go back to it?