A CTF can be hosted online, onsite or both. Online CTFs can basically be made available to any player with Internet access. The hosting of online CTFs can best be done in a cloud environment such as Amazon, which enables for easy and quick deployment, adjusting the resources where needed and no need to buy expensive hardware.
Onsite CTFs can still be hosted online in the cloud, and then only be made available to the players onsite. Alternatively, the CTF can be hosted on systems at the onsite location. Hosting on the onsite location takes away the need for a strong internet connection but does require more hardware onsite.
CTFs can be single event or can be split in a qualification round and finals (or multiple qualification rounds and a final). Single online CTF events in general take around 36-48 continues hours. Single onsite CTFs are generally hosted for one day (8 hours) or a set of days.
Choices can be made in the amount of time the CTF needs to run, number of players, the amount of challenges, difficulty of the challenges and the categories of the challenges. Further factors to consider are the preparation time, individual or team based and the exclusivity of the CTF challenges.