I teach for the most part privately in Paris. Don't feel that this is an obstacle if you don't live in France. There are very practical ways to resolve this apparent difficulty.
For local students lessons are usually weekly and last one hour. But many of my pupils come from further away, from other countries, and often come for intensive study over a few days or a week, sometimes choosing to take two lessons a day. They then repeat the process maybe two, three or four times over the year. This is the ideal solution for singers already started on a career. The system works extremely well, and I have seen immense progress made in this way. The “total immersion” effect is stimulating and my pupils and I love this way of working. Students record their lessons and then have a solid amount of material to work through once they are back home. We keep in close E-mail contact, and can discuss in depth any questions that arise. I also travel to give master classes and courses, and often renew contact with these pupils in this way as well.
I take students from conservatoire level upwards. I will also happily take anyone with evident vocal talent, but they should be reasonably prepared musically. I only teach adults.