As a general rule, a PowerPoint presentation with graphics and audio should not last more than five minutes. There are several factors that support this guideline.
First, your presentation will have to be downloaded by students using a variety of connection speeds. Almost any connection can handle a few megabytes. Unless a students has broadband or DSL, however, the download time for a file can become a major problem.
Second, the cognitive style of PowerPoint suggests that its best use may be illustrating a concept, reinforcing an idea or providing an overview or summary. Remember that bulleted points do not promote critical reflection; they do, however, work well for summaries and overviews. Moreover, it is possible that to illustrate an idea, you will need only one slide placed in the midst of your lecture!
Third, good online teaching practice suggests that complex material should be "chunked" into smaller, manageable segments. Indeed, a good deal of the hard work of teaching is breaking a complex idea into smaller segments that students can master.
Finally, there is the simple matter of attention span. Placing an entire lecture online with accompanying bulleted points will not keep the attention of your students. In fact, a lengthy online PowerPoint is less likely to hold a student's attention that a face-to-face one.