You can start right away! My LO loves tummy time and will spend 30 mins or so on the floor or on the Boppy. Some babies don't like it. If that's the case, don't force it. It's not an absolute. I know a mama who made her LO lay on the floor and scream through 10 mins of tummy time each day bc the dr at the hospital told her that babies need 10 mins of tummy time daily. Do what is comfortable for your baby. Trust your gut.
They recommend tummy time once you get home from the hospital but not for very long. Mines didnt completely start until 2 months he didn't like it at first.