As a teacher I always answer whatever messages drop in my inbox and I always try to be quick with my reply. I used to do a bit of customer service, so I guess my background helps me understand the importance of that contact.
That said, I recently stumbled upon this little tab called "potential students" that I had never actually explored for the simple reason that I didn't want to be bothering people with messages just because an algorithm said so. My bad. What I found was a four-month-old list of people who had already filled out the contact form and had been wanting to take classes with me. For some reason those forms hadn't registered as messages (not anymore, I have been receiving them in my inbox now... I often check if they do).
My point is, teachers who (aren't clumsy like me and) can navigate the website, will answer as soon as possible. Teachers who want to (and have the time to) provide good service will answer your messages as soon as they drop. Reply to Contact Forms? Of course, IF we see them. Also, with the 'automatically accept lessons' option, teachers get the notifications as an email... Which I check maybe once a day. And time zones, well, it's 10am for me, maybe most of you are already sleeping and I'll get a reply to this in 12 hours?
One last thing, try to keep in mind that teachers and tutors get paid by the hour, any extra time they pull on iTalki won't be compensated. So maybe it isn't that they don't want to answer, and actually they're busy with their other paying job?
Anyways, I can only speak from personal experience and my point of view (as naive as it may be). Hope this information was helpful. Have a great day everyone!