"It's time for your annual holidays again, and there decisions to be made. Where to go? How much to spend? ..." This could have been written grammatically, with punctuation, but it's common enough to see this. The difference is that the author is not trying to pass these off as sentences.
This author might even go into a post office, and ask, "How much to post this article to my publisher?" - knowing that this isn't a sentence either.
The example you've given heads the paragraph, and could be viewed as a heading. That is it's function.
An author writing an essay might write, "I sat down to write this article and I was immediately confronted with a problem. How to spell 'porpoise.'" This isn't grammatical, but this author, if they are a native speaker, will not log on to a forum such as this and ask "How to spell 'porpoise'?"
Student or Learner