This will vary quite a bit from baby to baby and from mom to mom. What's important is that you allow your baby to finish with one breast before offering the other. By letting baby control his feeds in this manner, he will feed most effectively and will help your body to bring in a good milk supply. Plus, it will ensure that he's getting just the right balance of fat and fluid.
Rather than focusing on how much time baby spends on each breast, watch what he is doing while he's nursing. Babies will often have periods of active sucking, followed by brief pauses or light sucking. When these pauses start stretching out and baby is no longer doing much active sucking and swallowing — or if baby comes off the breast on his own — then it’s time to offer the other breast.
Some babies, at some times, only take one breast at a feeding. As long as he is showing signs of being full and content, this is completely fine. Other babies might want to go back and forth between the breasts several times — this is also okay. The key is to watch your baby and follow his cues.
Most young babies will take somewhere between 20 to 40 minutes to complete a feeding. If your baby is feeding for much longer than this, or does not appear satisfied after nursing, then it would be a good idea to have his feeding evaluated by an experienced IBCLC.