What instrument is it neveragain? I think 35 mins is more than adequate if music is going to be a serious hobby rather than a career. It's also easier to practice for more time on the violin than it would be on some wind instruments.
Currently, my son spends up to an hour on the piano but I'm sure he could do it in half an hour if he focused and didn't decide to faff about or compose after every scale.
Bassoon will be 3x40 min sessions on a good day but sometimes is 1x30 min. There is usually one day a week, sometimes two, where he does not practice and I fundamentally believe it is important to have this break so the connections in the brain can be made. He doesn't practice on the day he has a lesson or if he's rehearsing for a couple of hours for a concert but he will practice on the days he goes to orchestra in the evening.
But it is all about practice. He went to music school in January with grade 5 and he'd been playing 3-4 times a week for 30 mins. He's now doing an average of 1-1.5 hours and is playing grade 8 pieces in less than two terms.