How much you will need to retire depends much more on your lifestyle and future medical issues than anything specific to Thailand, especially for younger retirees as costs of living tend to converge across locations over time.
Many people have got caught out by deciding to retire in Thailand based on overly simplistic calculations, and then end up having to move to even poorer countries as living costs rise or return home due to uninsured healthcare needs.
What's your nationality and age? Do you mean 1 visit to Thailand for 6 months or many short trips totalling 6 months? How do you fund yourself when you're in Thailand?
Bangkok starts becoming very trampy within a few miles from downtown, but the key point if you're not bound by work commitments is that you can time your journeys to avoid the worst of the traffic - just choose somewhere with easy access into / out of the expressway system, which is an amazingly fast way to get around the city, except during rush hours.