The short answer: As little as possible.
Text on slides doesn’t work. Bullet points don’t work. Audiences can’t read and listen at the same time, and so they read your slides and ignore you. Therefore, text should be kept to a minimum and you should instead use visuals.
When is it OK to use text?
In labels. Graph axes, for example, need labels in order to make any sense. You might need to label sections of diagrams. It might also make sense to label some images, such as those that are representing different types of focus groups.
In title bars. Slides need titles to provide context for the information delivered.
In quotes. When you’re using a quote or testimonial as proof, you need to have it on the screen word for word. This doesn’t, however, mean that you should read it out!
Names. It often makes sense to write the names of people, and brands and products/services when you can’t use a logo.
In your value proposition. Your five key benefits should be written down and repeated throughout your presentation, to reinforce your messages.
Otherwise, you should use images, graphs and diagrams to get your point across.
Worry about rules like ‘6 words per bullet points, 6 bullet points per slide’. This just focuses your attention on cutting down the length of your bullet points, and not on how you can effectively visualise your messages.
Make your text too small. Using too small a font makes the problem even worse, as if one thing will irritate your audience more than too much text, it’s too much text that’s too small to read.
Really, you should not think about your slides in terms of text. Replace text with visuals and engage your audiences rather than bore them. Your audience will thank you.