Depends on age
If your baby is breastfeeding
how long he is on the breast depends on his age.
In the beginning he's still learning how to nurse effectively and your body is still mastering the let down process. In the first few weeks each feed may take 40 minutes (10-20 minutes on each breast). By the time his 6 months old you'll both be more efficient and he'll spend 5 minutes per side and be back to playing.