A provisional application is by definition an initial application for a utility patent. It cannot be converted to a design patent. More accurately, it will not set a priority date for a design patent application.
In 2000, the average "pendency" was about 24 months for utility patent applications (non-provisional) and 18 months for design patent applications.
A more recent estimation for design patents is 16 months -
And for utility patents 32 months (it depends on the field of invention).
Utility patent applications (non-provisional) - unless non-publication is requested (if you are also filing for foreign patent protection, this is not an option) - are automatically published 18 months from a filing date. Design patents are not published before they issue.
There are three types of patents -
Plant patents - which protect the discovery of new plants.
Utility patents - which protect goods and services and aspects of them which serve a functional/"utilitarian" purpose other than, say, looking pretty.
Design patents - which protect the ornamental/non-functional design of a "utilitarian" product.
Plant patents are largely besides the point in most cases, utility patents are, in lay people's terms, what protect ideas, and design patents offer relatively narrow protection against knock-offs.
The protection offered by design patents is similar to the protection offered by trademarks and copyright, although the protection only spans a maximum of 14 years. Occasionally design patent protection can overlap with trademark protection (Google 'trade dress' for more info). There are also occasions where design patent protection can overlap with copyright protection, but copyright law, in the US at least, does not tend to protect the appearance of "utilitarian" products.
In foreign countries, design patents are often referred to simply as "industrial designs" or "registered designs." Protection is more likely to overlap with copyright law.