Books like When the War Began
John Marsden
When the War Began is 286 pages, about 5 hours 15 minutes at 250 words a minute.
8 books to read next
How long each takes
Skyward 9.7 h 
Grenade 5.0 h 
The Infinite Sea 6.2 h 
I Survived 2.6 h 
Parvana's Journey (The Breadwinner #2) 3.7 h 
The Last Star 6.6 h 
The Drowned Cities 8.0 h 
I Am Number Four : The Lost Files 2.4 h Bar height shows length. Estimated at 250 words a minute.