How long are you nursing per session?

At the hospital they said baby should nurse each side 20 mins then offer formula top up, and not to exceed 1 hour feeding time overall as baby will burn more calories at that point than intake from feeding.

I'm still holding to this guideline for my 5 week old but wondering if it still applies at this age?

How long are your feeding sessions and how old is your baby? Do sessions get longer or shorter as they get older?