start and timer and find out yourself :D.

As about me, all the process of preparing my contest (672 div. 2) was pretty long. You can divide contest preparation into three main parts:

1) You are inventing tasks. This is by far the longest part. You are trying to invent interesting problems.

2) Preparing the contest. I was preparing my contest for about a week. You need to make strong tests and good statements, so it is a long process. Then my round was tested by several testers.

3) The contest itself. Just have some fun writing an announcment and/or editorial.

Questions:

1) How somebody creates a contest? Invents some tasks and proposes them.

2) Do they have to think of all the problems? Yes, of course, you need to invent a unique task.

See this

Ranging from a few days to a few months. It usually takes a lot of time and effort to create a good quality contest. Original poblem ideas don't hit as often as would be desirable. Even so, the idea is just one part of it.

Authors have their biases, so testing helps identify more information about the problem, for example- a trivial solution that was overlooked might exist, making it much easier to solve. Maybe the author's solution was wrong, correction would be needed at that end.

Generating test cases — generating test cases requires thoughtful consideration. A simple random generator is only so good. Generating biased inputs, specially generated ones, small/large inputs, inputs considering corner cases. Wrong submissions are tested against the generated inputs, if they're passing, additions/changes need to be made. Multiple such buggy/incorrect/inefficient codes must be tested.

There's really a lot to problem setting and contest creation, more so for official, rated contests. The efforts of problem settlers, authors, coordinators, tested, headquarters, everyone who makes such beautiful problems available to us, just thinking about it all fills me with gratitude.