If you get expert, you will then take less time to get more milk. A pump, electric or manual, is helpful, though a lot of women learn to hand express and get very adept at this. Your midwife or a breastfeeding counsellor can teach you. Some pumps allow you to 'double' pump, which means you, express both breasts at the same time. There are a number of good manual pumps available these days, too.
Babies differ so much.
Tiny pre-term babies can only take tiny volumes, of maybe less than 20mls at a time. Older, stronger babies take a lot more. A very rough rule of thumb is that a feed is something between 100mls and 200 mls - with older babies taking more at any one time. You may need to express more than once to get large amounts. Breast milk is safe in the fridge for 24 hours and in the freezer for 3 months.