I'm guessing you have UK passport, in that case (and if you have not arranged a visa prior to arrival) you would get a visa exempt entry, this allows 30 days. This can be extended in country by 30 days at thai immigration offices. To stay longer in Thailand you can then "border bounce", giving you another visa exempt entry for 30 days (again extendable by 30 days).
You can have only two visa exempt entries by land border per calendar year.
So:
Initial arrival plus extension
30 + 30 =60
Border bounce 2 times
2 x (30 + 30) = 120
Total 180 days.
(If you have perfect timing)
You should note your airline may require proof of travel out of Thailand within the first
*****
day period.
You may also need to show various proofs of cash, accommodation, and onward travel for each of the border bounces to the Thai immigration officer.