How much time should I allow for a border bounce at Mai Sai?

^Latest major policy change was 31-Dec-2016 visa-exempt entry now limited to 2x per calendar year by LAND, but by air no stated limit, and it's 30 days by air or land (before most just got 15 by land). But yes you can now do consecutive visa-exempt entries. Back to back tourist visas also no stated limit of consecutive times, at the discretion of your friendly consular visa boss.

I thought Thai immigration said in 2016 that they wouldn't allow back to back visas and border runs made to get around immigration policies?

Has something changed since the announcement?

I’ve always got the 08:00 bus up and the 15:30 one back without any issues.

Two hours minimum, depends on whether there is a bottleneck at the immigration desk.

Two hours total.

Allow around 60-90 minutes for simple border hop. There is 8am bus arriving 1230 then one coming back at 1400, or take the next one to allow more relaxed pace.