15 mins sounds good to me. C is 9 weeks today and he feeds between 10 and 15 each time. He never goes longer than 3 hours during the day, sometimes 2.5. I has him weighed last week and he was 12lbs so similar size to your LO.
I have given formula (when I needed DP to do a feed and I couldn't express enough) and if didn't make him last any longer between feeds. He drank 6oz and was still up 3hr later so I wouldn't introduce formula just for that reason.
Hugs, it is tough some days/nights - sometimes I want to sleep right through and have DP do all feeds but I obv don't, I just persevere and know its worth it