How long should it take to prepare a presentation?
It depends on how new the information is to you, who you are presenting to and the importance of the presentation.
The rule of thumb is three hours of preparation for every hour of presentation.
But that is only a rule of thumb. If you are an effective presenter, know your topic and know the audience, you can succeed with very little preparation.
However, consider the example of my client who worked three days on a 12-minute presentation. As a result of that presentation he won a $10 Million contract. He already knew his topic and the audience. But he was willing to invest his time to fine-tune his delivery. Three days of preparation to close a $10 Million deal sounds like an excellent return on investment.
I think it was worth the three days of preparation.
If you want to deliver better presentations invest more time preparing.