You can go there at any time of the day, to have a Italian breakfast with cappuccino & croissants; in the lunch time to have some fastfood type pizzas or a proper lunch (since they serve also lunch with a rich buffet of the daily dishes) or for a afternoon snack a piece of cake & coffee.
Inside there is a bar that you can have your coffee and eat there or take away. But get ready for the crowd, as all special places in Milan; they also have a lot of visitors who are impatient to taste the wonders of Princi. My favorites are the cheesecake and the brownies, which are super-delicious, but also the salty species are worth trying.
Princi is quite close to the Duomo Square, has a central location. There are 4 branches in some other parts of the city, as well another branch in London. Prices are various. I recommend you to ask before choosing.