Within one or two lessons with a new singer, most of the time (I have had a exceptions) I notice a singer will make an improvement by the second lesson. Improvements in singing come down to specific, physical, movements. These movements, though they can be discovered quickly (example: want to sound more nasal? move more air through your nose while you speak) you have to repeat them, and be able to tell that you did it correctly without needing a voice teacher to confirm it. Then you need to do it long enough for you to be able to call upon them at any moment (want to be able to sing an entire song maintaining a nasal sound? You may need to practice that, and don’t laugh, that’s part of what made Elvis’ signature sound!).
Too many first time singers forget that they are learning an instrument, and instead think of their voice as if it’s their hair. They go to a hair dresser, get it cut once, and it’s somewhat fixed into a good style until it grows out in a month and they have to come back. Not quite that quick or fixed! If you are going to take voice lessons, make sure that you have committed a set period of months to practice and pursue this interest, with your goal being small but steady and specific improvements. Then you’ve got something that lasts much longer than a month, more like a lifetime. That’s definitely a time investment worth singing about!
So in general, especially for beginning singers, 6 lessons taken in one month is better than 30 lessons taken over 3 years. Those 30 lessons are likely the same lesson, happening on repeat, because there wasn’t enough lessons in close enough proximity for new vocal coordinations to stick.