That depends on how determined I am for a good yeild lol!
Mae is still a single sided feeder so I express on the side that she's not feeding on. Sometimes, I'll continue expressing for up to ten minutes after she's stopped (she only feeds for five minutes and often the pump is still getting milk at this point). If that's the case then I'll often try the side that she's just fed on too - I don't normally get much but there's often a fresh letdown.
When I was pumping to boost supply with DD1, I'd pump 15 minutes on each side after a feed.
HTH