How long is the visa valid for when returning to Thailand from Cambodia on a British passport?

He wont get a visa but he should be stamped in for 30 days visa exempt

If he Doesn’t buy a visa get will get another 30 day visa free exemption stamp which he can extend at his local immigration office for ฿1,900 for a duration of 30 days giving him a total of 60 days in country…

you get 2 land border crossings per calendar year (01 Jan- 31 Dec) and there is no official limit of arrivals by air but eventually you will run into an overzealous immigration officer that says otherwise….

if he purchases a visa in Cambodia let’s say a 60 day SETV for ฿1,900 at the Thai embassy/consulate he will be stamped in for 60 days which he can extend at his local immigration office for a cost of ฿1,900 for a duration of 30 days giving him 90 days in country…

With the visa he will not be charged one of his 2 land border crossings per calendar year (01 Jan - 31 Dec)

good luck

He will get another 30 days returning to Thailand which can be extended for 30 more days 1900 THB. 60 day TV from Cambodia.

Every time you enter you get a new 30 day stamp if entering visa exempt.

A tourist visa that you purchase from a Thai embassy before going to Thailand is valid to be used for 90 days from the day it's issued.

Once you enter Thailand with a tourist visa you are stamped into Thailand for 60 days.

Both visa exempt and tourist visa can be extended one time at immigration for 30 extra days.