How do you decide which events deserve full scenes, as opposed to narrative summary?
The quick and dirty answer is anything that arcs character and moves the plot should be done in scene so that the reader can see it and be part of the journey. Anything that doesn’t arc character and move the plot should be summarized as briefly as possible or just cut.
Let’s pause for a moment and talk about summary. Summary means condensing information down to the shortest possible description instead of playing the event out in front of the reader. Summary covers a lot more time than scene does because unless you’re writing a novel that takes place in three hours, you’re not writing in real time. There’s that eight hours of sleep your characters get every night. The time they spend in the bathroom. The time they spend at work doing routine things. Taking out the trash. Filling the car with gas. Unless any of that has a direct bearing on the story–the bad guy comes up and flicks a match into the tank while they’re filling it–you don’t need to write it and in most cases you don’t even need to summarize it.
That’s because most of normal life is an unmarked state, the stuff that usually happens, so the reader will assume it for you. You write, “The next morning, Jane left for work . . .” and the reader assumes Jane had a good night’s sleep because you didn’t say she had a lousy one. A lousy night’s sleep would be a marked state, and marked states have to be put on the page because they’re counter to the reader’s assumptions. This can be a little difficult if the marked state is something the POV character is used to (which makes it unmarked for her) because you’ll have to find a way of putting it on the page even though the story’s in her POV and she’s not noticed it. If your character gets up every morning, leans out her window, and shoots a pigeon, that’s unmarked for her, but marked for the reader, so you’ll have to write something like, “Jane leaned out the window and shot her morning pigeon and felt immensely cheered that she got two with one shot; that hadn’t happened more than twice in the ten years she’d been picking them off.” If after that all she does is get dressed, eat breakfast, catch the bus to the office, take the elevator to the twelfth floor, say hi to her secretary, sit down at her desk, and hire an assassin, your next sentence can be, “An hour later, she was still cheerful as she sat at her desk and surveyed the vita of the latest applicant for the assassin job.” I’d guess that 90% of all summary is showing time has passed. Do it in a phrase, not a sentence. It’s a marker, not part of the story.
So the rule of thumb is, if you can take a scene out of your story without damaging your story, just summarize it in one sentence or cut it and move on.