I have 15 wk old twin girls who take anythin from 45 mins to an hr & a Half to finish a bottle is this normal?

There on 180 ml with 6 scoops which is right for there age according to the guidelines on the back of the milk, they fidget cry an wriggle about during all feeds, it's a complete nightmare and it's all I do all day, they are sick after feeds too.. They do sleep throu 10pm-7am & were just under 8 wks prem, they couldn't suck when there born so I expressed for 3 weeks then my milk disappeared & they've been on aptimal since ..