If you are going to visit Bangkok and finding the accommodation, you will see that some places offer you both air-con rooms and fan rooms. And of course, the fan room is cheaper. If you are a budget traveler, you might have a question. Is a fan room ok? Or should I pay more for an air-con room? Here is my recommendation about this.
As a person who lives in Bangkok for almost of my life, I would say Bangkok is usually hot and humid throughout the year especially from March to May (some years until June). If you come at these months, I would recommend you to pay more and go with the air-con room. The temperature does not go down much at night. Pay a little bit more and you will have a nice sleep.
For the rest of the year, it is not hot like those 3-4 months, but it is still not cold. One more thing that you have to consider is the mosquitoes that try to sneak into the room when you open window; and some bad quality mosquito nets do not help much. So, my recommendation is still to choose an air-con room even if it is more expensive.
For me, I turn on the air conditioning every night for almost the whole year. Only some nights in December and January I turn it off and use the fan; I would say not more than 10 nights in a year that I do not use air conditioning.
Hope this information helps.