Coordinating a military coup is very difficult and takes a great deal of planning. It is almost impossible to determine how long it would take to organize one, as it depends on various factors and how fast organizers can secure them.
Here are the principal factors:
- The organizer must be sure that their actions will gain the support of many political actors and a large part of the population.
- The organizer must gain allies within the military elite, police and secret services. When the coup attempt takes place, units will be needed to perform various tasks. Units from all three institutions are paramount.
- The trained units from the police, secret service and military need to have a clear plan of how to take control of key locations: military/police barracks and stations, army headquarters, transportation points (airport, subways, train stations, bridges), mass media headquarters, power stations and the residences of your opponents (and where their family, friends, co-workers are). Also, the units need to be able to apprehend their opponents at the very first opportunity.
- The organizer needs to make sure that the secret service will not catch on to the conspiracy. This is another reason why agents within the institution need to be brought to the cause.
- Foreign and internal support is needed in order to gather massive amounts of money that would be used to buy weapons, vehicles and military apparel. The money would be used for bribery as well
- The organizer needs to figure out how to neutralize mass media outlets that will take the side of the government and at the same time take control of some of its own news stations and websites. Propaganda will be launched against the attempt, so it is imperative to have counter measures set up in advance.
- The coup attempt needs to be executed at the right time. The people will legitimize your actions, thus, they need to resent the government and believe in the perception which you are trying to enforce.
- The organizer should attempt to keep blood spill to a minimum. For this, the units need to have a well thought plan for multiple case scenarios. If the support of the people is lost, then your chances diminish considerably. Also, family and friends need to be put to safety in order to avoid them being captured and being used against you.
- A manifesto needs to be drafted and presented as soon as possible after the start of the coup attempt in order to engage with the public and give an explanation for the madness.
- The organizer needs to think of a modality to share power after they have obtained it. A transition to democracy is the safest bet, and the organizer could gain leadership after seemingly delivering the power back in the hands of the people.
- Also, the ability to turn off social media sites and mobile networks would surely help if it is a military coup. If the people rise up to take down the government, then probably the government would attempt to cut the networks.
As you suspect, it would take a very long time to prepare a plan that would take all of these factors into consideration.
Here is a great list of coups and attempted coups: