I agree with HotI in the fact that taking that long at her age is definitely not out of the norm, both my girls would generally feed from 15-30 min on each side, it would avergae about 45min for me to feed them and that didn't reduce until closer to 6mths.
DD1 was a 'lazy' feeder and would fall asleep at the boob so we did the following: fed her undressed (with a heater on of course), if she fell asleep i would sometimes run a cool cloth over her, would change her in the middle of the feed to wake her up - this assisted her in feeding and to stop falling asleep but as previously mentioned she still fed for longer periods.

I personally would drop the bottle, lay in bed and feed her and see how you go - i know it's hard with other kids to look after so it might not be as éasy'as that but i'm sure you will find something that works for you.
Have you thought about seeing an LC to check if there is something different with her attachment or something?
I've prob been no help but wanted to let you know its not uncommon and your prob doing everything perfectly right anyway.