How often can a UK resident apply for a 60-day tourist visa for Thailand?

Are you talking about a visa or visa exemption?

There's the new DTV visa...5 year, multi entry. 180 days at a go....

As often as you like until IO stops you because your doing too much .

I come every 10 weeks and stay for 4 week and stay 7 weeks at Xmas .. been doing that for about 3 years now .

They have only talked to me once

You can fly in, just ask your girlfriend to be prepared to answer the phone 🙂 The immigration will stop you and ask you why you are flying in and out. Tell them your gf is thai. They will ask you to call her... will work for a while. after a few more times. they will ask you : get marry.... (that is why i got married.... ) ((take more pics of u 2 together, might help if you get pull over.))

You could enter visa exempt for 60 days and extend for 30.

You can do it until you can't. The embassy in your home country will probably issue you as many as you want.

That doesn't mean you can enter Thailand just because you have a visa. It might improve your odds, but you cannot live in Thailand, or even remain in Thailand long term, without a long term visa. Eventually you'll be pulled aside at the airport when trying to enter and asked what you think you're doing. You might receive a warning or you might be denied entry. Think about what the travel patterns of a normal tourist would look like, and if what you are doing doesn't match that, it will be obvious to the immigration officer.