You can start as soon as your supply is established—usually around 3–4 weeks if you’re nursing and don’t need to pump yet. If you’re exclusively pumping or combo feeding, you might start earlier.
Use clean bottles or bags, label with the date, and store in the back of the freezer (not the door). Fresh milk in the fridge lasts about 4 days; frozen can last up to 6–12 months.
Just don’t panic if your freezer stash looks more like three sad ounces than the milk waterfalls you see online. You’re doing great.