I usually start with a prebuilt template these days as you can pick up one relatively cheap (and dirt cheap in some cases). Then build and "kit-bash" and modify to your hearts content. When I design cookie cutter web sites for people, I can be done with a 5 page site in less than 3 hours when they provide the text and images they want online, up to 5 hours if they only provide a logo and text, and it could be DAYS if they only provide a logo with nothing else.
I think Connor encapsulated the process of a web designer very well in his example, and it is generally the process a professional web designer will take. Firstly asking a client what they like/would like (thus example sites, ideas etc) combining this then with what they have provided and then gluing the lot together both in code and aesthetically.
I find more and more that both designers and clients don't start from scratch be that in terms of their requirements, layouts or styles, this is mainly due to clients wanting roughly similar sites to their competitors with only a few distinguishing features.
Thus why reinvent the wheel ? (us designers don't) meaning as Connors example, times vary both on what resources you have be that memberships to sites with prebuilt resell-able templates, PHP Class libraries (Zend, Pear, Smarty, Custom?... etc...), CSS files and documentation and even reference books (useful at times
).
As well as provided time frames/deadlines, manpower and mainly complexity. Personally I can turn around integrations within 30mins/1hour, complete CMS templates within 4/5 hours, basic sites much link Connor and upwards to 2/3month team project/CMS/Service builds.
However i have over 8 years experience and am yet dont consider myself an expert, as such I would recommend much like a developer you set yourself or ask for say a couple of easy and more difficult briefs (much like a college/university web design course) and set about completing them.. then you will get a better feel for the time involved at your level of knowledge and with the resources you have.