You are in the UK? So, the process is very easy. I take it you have the appropriate funds behind you in the UK? Within 60 days I believe you need to go to the Official Royal Thai Embassy Online Portal:
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Fill the application in online for a Non-O multi entry or single entry whatever you prefer. There's no Police check on this application. You need to upload a photo, passport, bank statement as a JPEG. You don't even send in your originals. If you fill it in on a Sunday send on a Monday you will be approved within 10 working days.
They will send your visa approval online just print off. You can then buy your flight and health insurance.
When you arrive in Thailand eg BKK port immigration will stamp your passport for 90 days. This will give you and your GF time to get the paperwork needed in Thailand to convert your Visa to a Non O. When you arrive where you are going to stay. Go to immigration and register your address. This is really important for your visa application.
DM if you get stuck but The Royal Thai Embassy London has top staff and is always willing to help. Good luck
If it's for the UK she can stay for the entire time of visa. Start date stamped in visa to end date. If she spends more than 8 weeks I believe out of the UK within the visa period. Her visa will be revoked. You need to check this with the immigration officer at port of entry as the law changes by the week.