I wasn't really sure what forum this belongs in, but After 1900 seems the most appropriate. What do people figure is the shortest possible time that a dictator can build themselves up to Stalin or Kim Il-Sung levels of adulation from the people?
Could be done in a year or two with backing. Cults can be built very quickly, with a strong core in just months if the cult-leader knows what he/she is doing. Timing and social/political situation is critical. If the society is going through some kind of disaster, it's very fast. If the core of followers are also highly intelligent, skilled writers & speakers & salesmen, and includes people with the political/government/military connections....
Of course, it does depend on how clever the leader is, and how close to the truth their words strike home. It's important to have a core of very determined followers/allies with similar aims.
First they have to encourage doubts and questions about the previous paradigm. Then start attacking it until a large percentage of people lose trust in the old paradigm. Then offer the possibility of something "better", not spelling it out too much, letting people fill in the details with their own assumptions.
I'd say 2 years is plausible, once they really get going. They may have to wait several years for the opportune time however.
If they take too long to build up, they may have missed their chance and other factions may copy some of their ideas and ride off with them instead. Just like politics or business. It's rare that there is only "one" such group with a single leader.
Oh, also the CULTURE of the society can make a big difference. Some are definitely more susceptible to certain influences and dominance. Every culture has hot buttons, just have to find the right ones.