ideal would be 1.5-2 hours. but 45 minute naps are common at this age and can last for a while. if you have the energy, it would be good to try to extend the 45 minute nap, as good day time sleep begets good night time sleep. Unless that is if your lo is waking up happy after 45 minutes, then leave them be. chances are though that an 8 week old after a 45 minute nap isn't too happy?!?!