Congratulatiosn on your ds. It sounds fairly typical behaviour . You can't really ration a newborn as even comfort sucking will promote your milk supply and soothe him. He may well be feeding , resting then feeding again during those 40 minutes but that is perfectly normal as his tummy is so tiny he will digest each feed quickly.
i do agree. however, in the early days for me, i had such painful nipples, that i did take ds off after 50 minutes for a 30 minute break, to then go back on for another 50 minutes! he fed from 6-midnight every evening and i know this is normal, but he was making me sore. you can also tell by looking and seeing if he is swallowing/muscles above ear are moving. congratulations btw