Breastfeeding time

What's your regular amount of time you breastfeed at once? My son doesn't really feed longer than 20 minutes, and he doesn't seem hungry until a few hours later, but I'm worried that sometimes he doesn't feed ling enough. It's hard for me to tell simce idk how much milk he's actually drinking. I would like to see if the time he feeds is an average time, or if he's not feeding enough