36 to 90 hours to prepare a 60 minutes presentation with about 30 slides. It is not a rule, but an advice from Nancy Duarte in her book slide:ology. If you don’t know her, Nancy is a communication expert, known for supporting great speakers. One of her most acclaimed job is the Al Gore famous presentation on climate change.
Going back to the time you need to prepare a successful presentation, it is difficult for me to give a precise value, you need to allocate many hours if you want to achieve good results.
A common objection I get is: “I don’t have time to prepare, because I am too busy”. Ironically sometime that busy is about delivering presentations itself. Do you think an actor would take the stage unprepared just because he has a full schedule of shows?
If you really look at it the criticism comes from the approach. People who think they don’t have time is because the time for preparing is perceived as a low priority. Professional speakers. would they be actors or great corporate speakers like Steve Jobs was, value preparation and allocate the right amount of time (a lot!).
In conclusion: how long should you prepare?
There is no magic number, the only thing I am sure is that a successful presentation calls for an adequate preparation. That requires some time. You need it to design your talk, research the subject, outline and organise the content, review it and rehearse it. Those are all very important steps and you cannot cut corners or avoid them without putting a dent in the final outcome. If you don’t have enough time the best solution maybe to postpone your presentation!