You can get a tourist visa to the UK for that length of time. Maybe extremely difficult as you are only recently started the relationship.
The main criteria is proving that she has a reason to go back to Thailand.
Obviously you have to prove it is a substantive/serious relationship, and not a holiday romance.
Could suggest the use of an agent, as they deal with these things all the time and know the ins and outs of what is required.
@Key ***************
is a good company, they handled all my previous fiancé and settlement visas.
Also other companies like
@One *****************************************
are also good.
Many will suggest trying doing it yourself,which you can attempt to do.but you will have a higher percentage of refusal, as not knowing all the requirements for a successful application.