Sometimes we get blinded by the day-to-day grind we fail to see improvements we've been making. I bet if you compared a recording before you started practicing Long Tones to now you'd notice a difference. Long Tones are your Wheaties and you gotta just keep eating, every day. When I was an undergrad I felt the same way you did at the 6-7 month mark - "these exercises are pointless, I don't hear anything different". At 9 months the college band director told me he noticed SUCH a marked improvement in my sound and at 12 months I won a scholarship for the next year. Just keep at it and it WILL pay off. Maybe one day try them at a different dynamic level - try to use Long Tones to improve an aspect of your sound you don't like. For me it was a thinness in my sound and I just played long tones until I could sustain without that thinness. I also had a problem with consistency - I'd start a tone ragged and couldn't hold it for 8 counts with any sort of consistency. So I just started working on that, every day trying to be a BIT more consistent. Some days it felt like I wasn't doing any improvement but only when I had juries the next year and the jurors all said "WOW you've been improving so much! Such a difference!" - And really all I was doing was long tones and Rochut.
Adams E3 0.6 with SS Bell
K&G 3.5D
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Founder and Solo Euphonium
San Francisco Brass Band