So far:
The prologue took place over the course of like... a couple of hours. You can see the sun set and it starts going dark as it goes.
Then there's a TEN YEAR TIMESKIP
Chapters 1, 2 and 3 happen across the course of a day. Fun fact! I had to work out what time it was because you see people's phones a few times in these chapters...
So we start with Subo at mid-late morning (around 10-ish?), and he goes looking and meets Rekki. We get a flashback to Rekki earlier that morning, showing Rekki is up and ready to work at 7.30 (god, Urien is a horrible boss, isn't he?) So she got up, went into town patrolling with her support knight, who "went for coffee", never came back and is seen handing in her resignation to Felix when Subo enters the Chapter House, leading to Felix running into the lounge a few minutes later to find a replacement and Subo getting the job.
Then after patrolling a while and beating up the studded chaps demon, Rekki gets messages from Jules...
It's now half past two.
And then after finding the anomaly and witnessing Jules' amazing application of "delicate science, and seeing the really big demon emerge, Rekki messages Sarin at...
Twelve past three.
So "book 1" (chapters 1-3) outside of the prologue (chapter 0) spans most of one day up until around teatime.
Chapter 4 starts up the following day and we know exactly when it is because good morning, Rekki!
And we're currently a number of hours later. Rekki has been to Camelot and back, Subo has gone to work and gone on an adventure with Jules and they've all driven to the University. It's afternoon now.
BUT Chapter 5 also had a lot of flashbacks, so it technically kind of also spanned ten years in a way, because we flahed back to days after the prologue, and then checked in for the following few years after it.
So in a roundabout way the answer is that Errant kind of spans an afternoon/evening and then two days, but it also spans ten years