I think this attitude of making sure each junior engineer has someone senior answer every question as soon as it comes up is doing a disservice to our junior engineers. One of the most important qualities of a senior engineer is that they can independently solve an open ended problem, especially one they haven't solved before.

Being senior means you have learned how to learn. You don't get this skill by asking a bunch of questions, you get it by answering your own questions without having someone else to rely on. This is a skill we should foster in our junior engineers as early as possible, and being available as a shoulder to tap on every 15 minutes actively prevents that skill from developing.

I think it is perfectly resonable to mentor a junior developer using a single, daily video meeting of 30 minutes or 1 hour. This is a time they can use to ask questions about their previous day, bounce ideas, etc. If a question comes up later, they can spend the day trying to solve it and report on it in the next daily meeting.

The best thing you can do for your junior engineer is to find a stream of work which challenges them at the right level (not too much, but just enough), and let them go independently while keeping some guard rails. This should not require many hours of 1:1 face time.

I understand this approach might not work for some juniors who can only learn in a highly social context with a mentor in the room. But I think it can work for many juniors, and actually be beneficial in the long run.