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.