I have to leave home to read a book. Sitting on a park bench is the only way for me to focus and not get distracted. It’s great, though. We have a beautiful rose garden nearby. Lots of critters scurrying about.
This is why I tend to keep my phone physically far away from me, and out of sight.
Anecdotally, I'd give it 3 months of "reduced phone usage mindfulness." For further reading, check out the wikipedia article on ∆FosB expression, which is a gene expression which essentially tells your body to keep doing things that release dopamine. It takes about 3 months for the ∆FosB expression to decay.