Question : How long do most people visit Bangkok?
Answer : Of course you're not most people, but if you were, you'd stay roughly 22 days.
Question : Is there another local airport that can get you to Bangkok?
Answer : Departing from London Luton Airport is not the only way to get to Suvarnabhumi Airport. Try looking for flights from London Heathrow Airport (LHR) as well. Over the next two months, flights from LHR to BKK are being priced 65% cheaper than flights from LTN to BKK. Being only 29 miles away, it's easily accessible for most.