Babies keep suckling for hunger and for comfort as well. During growth spurts too, baby keep suckling for longer duration and more frequently. If baby is gaining weight appropriate for its age, passing 7-8 times urine and 3-4 stools per day and baby looks content at the end of feed, probably your milk is adequate for him. If not you should probably consult a pediatrician. To prevent sore nipples, try releasing the latch of baby with the tip of your little finger slowly and remove the nipple after 30-35 mins at one breast. You may use nipple shield if nipples are too painful.