What type of visa do I need to retire in Thailand and when should I apply for it?

Non Imm O visa based on being over 50 years old, commonly known as a 'retirement visa'.

If planning to retire, get the 10 year LTR or 10 year O/X visa. Saves you a lot of ongoing time, money and effort to constantly renew at immigration, maintain financials etc.

More possibilities. I would say get a non imm. O Single Entry in y'r home country (90 days) based on retirement. And then after e.g. 60 days here, ask for a year retirement extension. Then also ask for a re-entry permit.

Also you can come in with a Tourist visa and ask for this non imm. O 'in country'. And THEN ask for a year extension based on retirement after e.g 30 days with a re-entry permit.

It's even possible to get this 'in country' non imm. O visa when you come in with visa exempt (So NO visa). This gives you 45 days (entering before 31st of March) to get this in country non imm. O visa.

For the year extensions and in country visa, you need at least have a Thai bank account with 800k฿ on it, (2 months before asking the year extension. Money transfered as international transfer for the in country non imm. O, no seasoning needed) in your name only, for reason of retirement.