being a Greek passport holder you do not get a visa on arrival, you get a visa exempt entry stamp. If that’s what you were told, purchase a tourist visa from your local Thai embassy/consulate.
you asked how long can you get on a visa, there are numerous visas you can apply for, as I said some details are required for people to help you. We cannot guess what you might be doing, it is not going on like a 10 year old boy as you said. But what you asked is like asking how long is a piece of string, or how much money is required to visit Thailand, the devil is in the details, so please be more specific when you are requesting some information.
How can anyone answer that, what is your age, any preexisting conditions, what is your budget, and what deductible are you happy with, there is no one fits all policy.