I had a studio apartment in Bangkok 5 minute walk from the BTS (rail system) cost was 15,000/month. Food I spent on average 30-40 baht per meal 3-4 times a day. Transportation 50-250 baht per day. Medicine, massages etc 5000/month.

If you're going somewhere like Issan, Northern Thailand or any smaller non island villages you can cut the rent in half and food and services are generally a bit cheaper.

Vices? That is for you to decide.. alcohol, tobacco and any naughty fun to might get upto will make everything more expensive. How long? Depends.

Visa is a real problem. Covid tests? I don't know the current status on any of this but it the visa and potential visa runs can cost a lot and I don't mean just money but time.