How long is normal for LO to stay on each side?

hilsy85

LO is 6 days old and EBF. I usually feed him on one side per feeding, and I try to just let him stay on until he unlatches himself. Sometimes this an be up to 40 minutes. I've heard that babies shouldn't stay on for more than 20 min because then they're burning more calories than they're taking in? Is that true? Is 40 min o a side too long? Sometimes it's less, but 30,35 min is not uncommon. Thanks!