It’s certainly different for everyone! I’d say chapter books are a lovely addition when an early reader begins to hunger for event sequencing, cliff-hangers and “What’s next?” prompts in the picture books they’re accustomed to. With that said, chapter books are ideal for read-alouds ANY time! We’ve been reading Winnie the Pooh over breakfast and my 2-year-old is just as happy to hear the story than his Piglet-loving older sister. :)
Feel free to take it slow with a few paragraphs at a time, and if attention span is lacking, simply re-introduce a few weeks or months down the road! We’re big fans of taking the long road, slow and steady. :)
My 4-year-old loves the Magic Treehouse series by Mary Pope Osborne. There’s adventure and cultivating a learning through books built right into the stories. We’re on book 9 and there’s still 19 more to go in the series, so plenty to enjoy!