And how much time does it usually take? I'd like to create a poster without having to put too much time into formatting.
I recommend the free and open-source desktop publishing program Scribus, available for a wide variety of operating systems including Windows, Mac, and Linux. Unlike many of the programs suggested here, Scribus is specifically focused on tasks relevant to poster production: layout, typesetting, and high-quality printed output. For example, Scribus includes a "Preflight verifier" to warn you about potential problems with your exported PDF file before you waste €40 on a messed-up A0 colour print. I've used Scribus for several conference posters and it's always produced good results. These days I personally tend to use LaTeX with the tikzposter package, but I know that some people prefer interactive, WYSIWYG tools for poster design -- and in those cases, Scribus would be my first choice.