Hi ladies! Good on you for getting onto a routine! The first little while can be a little daunting, but after a week or two, things should settle down. With regards to how long LO's eat for, they all vary. My DD used to take 15-20mins, but my DS (who's just 4 months corrected age) is done in less than 10mins. Like Fiver, we're on a 3.5ish hr routine as he's not ready for 4 hours yet. Your LO will tell you if they're not getting enough and it may take a few days, but if you ensure they've had their fill at E time and do what you can to 'stretch' them until the next E time, your supply will respond accordingly.
My DS is really distractable during feeds ATM and I need to make sure I give him ample time to feed, ie feed one side, burp, offer the second side, sit up, burp, chat and look around, maybe nappy change and then offer the breast again. So even though he may only be sucking for less than 10mins, E time may take 20mins. If you know a burp or other distractions hasn't kept them from eating all they want, it shouldn't be too long before they adjust to 3 hrs between feeds. Good luck!