I normally work about 10-12 hours for an hour's presentation.
I take the following factors into consideration.
1) Details of the proposed audience like, mix of gender, age, qualifications, experience, tech savvy or not etc.
2) The real subject of the presentation, the objectives.
A combination of the above decides my presentation.
I ensure that there would be enough pictures but not too many.
Certainly not too many sentences (though I confess that initially when I was experimenting with PPTs I used to realy write paragraphs no end)
And for an hour's presentation I prefer to have no more than 25 slides at the most but not lesser than 20 slides.
Most importantly I try to really choose the right PPT template that would compliment the subject, the objectives and yet can retain the attention of the audience.