In reply to a similar question, Abraham Lincoln said "Long enough to reach the
ground."
That's how long your legs should be. The presentation should be long enough to
reach the end.
Seriously, there's no way to answer that question without more information. If
the presentation is about "How to contact me", it could be one slide. If it's
about "How a beginner who can't even spell 'Visual Basic' can become an
accomplished programmer", you're talking about days worth of slides.