My baby only suck for 5mins?

My son is 4 1/2 months old and still does that. He is just very efficient with that one side and gets a lot fast and does not want the other side most of the time. He will let me know if after that side is empty, he still wants more but usually he just takes the one side for a few minutes and is done. As long as she is gaining weight and still having enough wet dypers it should be fine.