What is the duration of stay for visa-exempt entries into Thailand, especially after crossing into Cambodia?

You’ll see various websites that state 15 days for a land border visa exempt crossing. That rule changed in 2016.

Now it’s 30 days whether by land sea or air. The only difference is that by land you only get 2 exempt entries per calendar year.

Russians have a different entry scheme called a visa waiver and that has never had the restriction of two land border entries in a calendar year. Indeed now they get 90 days on arrival (if they choose to) but that entry cannot be extended, nor can it be changed to any other visa type within Thailand.