Question About Breastfeeding/expressing.need Help, Please.?

Hi, I want to ask if it’s painful when expressing milk. I have never used a breast pump before. I am 28weeks with my second child and I would like to try and express some milk when my maternity leave is over. I believe in breastfeeding exclusively for six months.
How does it feel when one is expressing milk (any pain or discomfort?) and how long would it take to fill a 500ml bottle?

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