How to deal with APA in-text citations in long texts? In APA referencing style, the first in-text citation of a work by three to five authors must include all the authors, while the subsequent citations should only mention the first author followed by the expression "et al."

Does this apply literally to a long text such a thesis (in my case, 25 pages)? Or is there some kind of reset, say, for each chapter?
APA style is mostly designed for defining manuscript submission rules for journal articles. 

When you seek to apply these rules to a thesis or book, you inevitably have to apply your own rules.

The general principle is that in-text citations should unambiguously identify an end of text reference. Thus, whatever principle you adopt should do this.

For a thesis, I think either approach would be fine. You could use et al across chapters or reset each chapter.

More generally, I think that if you have a combined reference list at the end of the book/thesis, then it's fine to not reset after each chapter. In contrast, if each chapter has its own reference section, then it makes sense to reset each chapter.