I submitted my thesis a month and a day ago. Although I understand that it may vary in general (I had colleagues who had their viva as soon as six weeks to as long as three months, sometimes four but that was due to extraordinary circumstances), from what I am reading, the expected viva dates for theses submitted during the summer months could be in September (more often than not, end or last week of September but I'm not sure if they are representative). The question though is, which part of the month is it most realistic to expect the date to fall in? Due to certain commitments I have planned, a late September viva seems fine for me, though first week of October seems best. I got a formal email from my uni's exams office that copies were sent to the examiners just last week (3 weeks after I handed it in) and say what if the internal examiner has not gotten it out of his letterbox yet. Are theses submitted to the designated point in July but only sent out last week (2nd full week in August) realistically headed for a late-September (at earliest)/early-October viva?

There is no standard timescale.
A friend of mine waited 7 months from submission.
My wait was either 6 or 7 weeks.
It took my own uni about 3 to 4 weeks to post my thesis to the external.
This phase was the worst part of my PhD.

What time of the year did you submit yours?

At my uni it can take up to 6 months for a viva. I think a lot of it depends on how fast your external is at reviewing it and then finding a suitable date. They are the biggest factor not the time of year.

I had a Master by research. In my case, the external did not reply for two months then uni has to assign a new external. After two months, you start to worry. It is still too early

Mu uni sent out an email before they arranged a date asking if there were any dates that were inconvenient for either me or my supervisor so they might do that and let you tell them when you're not available.

Lucky for you. Too bad based on stories I hear, I doubt that that is an option in a majority of institutions. Whilst I recognise that it's best to get this over with ASAP, I want to balance that with the ability to plan and not conflict with other commitments previously made. I also don't want to 'short circuit' the process by emailing the internal examiner directly (something which most institutions bar anyway) or even my supervisor and making the request myself.

I think there's a near 50:50 chance I may have overestimated the gap between submission and the viva itself.

I submitted at the end of March and my viva was in the middle of May.
My friend submitted in August and his viva was the following April I think.
I have seen other students in my old group having their vivas at all times during the year.
The quickest I have seen was a month gap from submission to viva.

The difficulty is getting your external and internal in a room at the same time.