Once you've submitted the thesis, how long does it typically take until you find out your viva date? I know both my examiners and they're keen to examine the thesis. I haven't heard anything so far (it's only been a week), but when would you follow up?

My internal examiner negotiated this and kept me up to date, it did take a good while though between submission and getting a date. It depends on your relationship with the examiners, but a week is really soon to be chasing it up.

Follow up with your supersivor: just make sure they organised the viva (as opposed to just forget about it for a few months). No names mentioned.
If they have sent the invitations to the examiners, you can't do anything about that other than wait patiently.

Probably not the norm, but I submitted at the end of September and didn't find out my viva date til December! and viva was not til January. My problem was my external examiner could only do Friday afternoon and my internal lectured on a Friday!

I think I left it a few weeks before I first chased up. At my uni viva was organised by admin team. I had to speak to them and was not allowed to approach my internal examiner about it.

At my uni we only arrange a viva date after the examiners' reports are in - and these typically take 6-8 weeks to compile. We have a non-examining chair on our panels who receive the reports and set up the viva. The candidate is only notified after a date has been agreed on etc. I hear the norm is about 10 weeks after submission, with around two weeks "notice" to prepare.